Primitive

A primitive, or prim, is a single-part object. Multi-part objects will have multiple parts ("prims").
In Second Life, virtual physical objects such as cars, houses, jewelry, and even less obvious things like hair and clothing are made out of one or more prims. Objects made from prims are usually created in-world using the built-in object editing tool. This tool is used for all sorts of 3D modeling in Second Life, playing the same role as 3D Max, Maya, or Blender, but customized for the Second Life way of doing things.
Each prim is represented by a set of parameters, including shape/type, position, scale/size, rotation, cut, hollow, twist, shear, etc. These parameters are sent from a server to the viewer running on the resident's desktop, where the local video card is used to render the visual appearance of everything. (Rendering on the server would probably create a much higher amount of network traffic.)
The color, texture, bumpiness, shininess, and transparency of prims can also be adjusted, and images (textures) can be applied to each surface (face/side) of a prim to change its appearance. Box, cylinder, and prism prim shape types can also be made flexible.
Prims can be linked together into link sets. They can also be attached to avatars, but this process is separate from (thought similar to) linking.
In the Viewer source code, primitives are implemented in LLPrimitive, with vertex generation performed in LLVolume and rendering in LLVOVolume.

The mass of a prim is a function of it's volume, different prim shapes have different volumes thus different masses.

Mass Lab demonstrates how different shapes have different masses.

The Basic Prim is a a single object. All prims start from one of the following type of prims: BOX, CYLINDER, PRISM, SPHERE, TORUS, TUBE, or RING, and can be altered in a number of ways after they have been created by using the Edit Window. All prims can be changed into any of the other types of prims in the Edit window.
The Edit window allows the direct creation of a few additional prim types such as a Tetrahedron, Hemicylinder, and Hemisphere, but these are in fact simple modified versions of the Prism, Cylinder, and Sphere respectively.
The type of prim called the sculpted prim is a special type which can be initially created with the use of the Edit window, however, it will not be covered here in the discussion of the "Basic Prim".
Once a single prim is created it can be Linked to other prims and become a Link set.

Building Tools

Focus Tool

This tool changes where the camera is looking in the 3D view. Selecting this tool will change your mouse pointer to a magnifying glass. Clicking any point in the 3D view will center that point.
Zoom - Using the scroll wheel or slider will zoom the view in or out.
Orbit - Click-drag will rotate the view around the selected point. This option can be selected temporarily by holding the Ctrl key.
Pan - Click-drag will slide the view parallel to the current viewing plane. This option can be selected temporarily by holding Ctrl + Shift.

Move Tool

This tool moves a selected object smoothly. Selecting this tool changes the mouse pointer to a hand.
Move - Click-drag will move the object smoothly in the horizontal plane.
Lift - Click-drag will move the object smoothly parallel to the current viewing plane.
Spin - Appears to work the same as Move.

Edit Tool

This tool edits the details of an object. Selecting this tool changes the mouse pointer to an arrow with a crossed axis symbol. Clicking an editable object will highlight the object with a yellow outline for the "root prim" and a blue outline for any other prims, and show edit axes and coordinate values on the screen. A slightly out of date introduction video can be found at YouTube.

Position - The axes are color coded: red for X (east), green for Y (north), and blue for Z (height). The axes are centered on the center of the object, which gives it it's "location" measured in meters from the bottom south west corner of the map region. An object is considered "in" a region or parcel if it's center point is within the boundaries, even if part of it overhangs.
There are small arrows at the intersection of the axes, which when click-dragged will move the object along that axis. There are two-color triangles near the arrows, which when click-dragged will display a grid and move the object within that plane.

Rotate - Also selected temporarily by holding the Ctrl key. Changes the axes to three rings. Click-drag on a ring will rotate the object along that plane. Sliding outwards will show a circular ruler, which will rotate the object in 1/64th of a circle (5.625°) steps.

Stretch - Also selected temporarily by holding Ctrl+Shift keys. Changes to small cubes on the sides and corners of the object. Click-drag of a colored cube will stretch the object in one dimension. Clicking the grey corner cubes will stretch the object in all three dimensions, with the opposite corner staying fixed.

Stretch Textures - When checked, will stretch the textures on the object. When unchecked textures remain the same size as the object changes.

Select Face -

Edit linked parts - Lets you select one prim of a multi-part linked object for editing. The single prim will be outlined when selected.

You can link several primitives (prims) together to create one cohesive object. A linked object is, for all intents and purposes, considered one object. It has one name, acts as one object (for example, if physics are enabled on it), and it cannot be broken apart unless you Unlink it. However, a linked object still counts as the sum of its prims when determining your land's object limits.
One prim of the object is considered the parent or root link. The name of the parent link is the name of the whole linked object. The inventory of the root prim is, for most purposes, the inventory of the whole object. The center (or origin) of the root prim is the center of the whole object, even if the root prim is not the physical center of the object itself. Vehicle scripts look at the root prim's orientation to determine the "front" of the vehicle. As a result, it is important which prim you select as the root prim.

Follow these steps to link together two or more prims:

If you are not in the object Editor already, right-click any object and choose Edit, or open the Editor with Ctrl-3.
With no object selected in the editor, hold down Shift and click on each prim you wish to link together, one at a time. Make the most important prim and/or scripted prim (root) the last one you select (such as the seat of a vehicle).
Then, go to the Tools menu and select Link, or just press Ctrl-L.
You can select Tools->Unlink or press Ctrl-Shift-L to break the object apart.

Be aware of the following limitations:

A linked object cannot exceed 54 meters in any dimension.
Normally a linkset can have up to 256 prims; sitting avatars count as one prim each.
Vehicles, or any physics-enabled object, cannot have more than 32 prims (sitting avatars don't count toward the physical prim limit).
There is no nesting of linked groups. In other words, if you link a third object to two objects already linked and then unlink them it will not yield two groups but three.

Ruler: - When the use grid option is checked, affects the numbers that appear on the rulers. World will show the region coordinates, Local will show 0 as the current location, Reference ???.

Stretch Both Sides - When checked, using Stretch on a single prim will extend opposite sides rather than just the one side.
Stretch Textures - When checked, will stretch the textures on the object. When unchecked textures remain the same size as the object changes.

Use Grid (Snap Option)- When checked, shows rulers parallel to the axis you are changing position for. If you drag mouse over the ruler, the object will jump to the nearest tick mark on the ruler rather than moving smoothly. If you drag near the axis line between the rulers it will move smoothly.

Selected objects: - The number of objects you have selected currently, where linked items are counted as one object.

primitives: - The total number of prims included in the selected objects.

Second Life's Undo is used to revert certain changes to an earlier state. While selecting an object, use Build > Undo, or the much quicker keyboard shortcut of Ctrl-Z.

General Tab

Name: - A newly created object has the default name "Object". This box lets you change it to any other name up to 63 characters long.

Description: - Blank by default. You can enter up to 127 characters of text here.

Creator: - Shows the original creator of the object, with a button to open their profile.

Owner: - Shows the current owner of the object, with a button to open their profile window.

Group: - Shows the group name assigned to the object. By default it is set to the same group as the active group of the creator. The Set... button allows changing the group assigned. The group assignment affects whether an object can remain on a parcel and who can use it.

Permissions: - Second Life has a permissions system to control how people use objects they did not create. The original creator has full permissions to copy, modify, and transfer the item, but they can restrict subsequent users from some or all of these abilities. Note: the contents of an object can have different permissions than the container, and copies may inherit different permissions than the original.

You can (cannot) modify this object - Indicates whether you have rights to change the object. Even if you cannot change it, normally you can take it in and out of your inventory, and if it is a wearable item, edit how it is attached to your avatar.

Share with group - If checked, any member of the group listed above can use the object.

Allow anyone to move - If checked, any user can move the object.

Allow anyone to copy - If checked, any user can take a copy of the object for their own.

Show in search - If located on a land parcel which is listed in search, the object will also be listed in that search result page.

For Sale - If checked, the object can be bought by other users. When checked, the price box becomes active, and you can set any price, including zero. The three buttons allow selling the original item, a copy with all contents, or just the contents and not the container.

Next Owner Can: - Sets the user permissions for the next owner.

Modify allows editing any part of the top level object's features with the edit tool except adding more permissions. The permissions of the contents may be different and not allow editing them internally, though they can be removed or added.

Copy allows making an unlimited number of copies of the object.

Resell/Give away (known as "Transfer" by most people) allows you to transfer the object to any other user.

When Left-Clicked: - Changes the default action when someone clicks on the object.

Touch -

Sit -

Buy -

Object Tab

This tab allows you to edit the size, shape, and location of an object. Most objects start as simple geometric block selected from the Building Block Type dropdown list. The default shape is a 0.5m cube. The Sculpt type is defined by a texture map and can be a fairly complex shape but still only be one prim. See this 3D modeling guide for detailed information.

Locked - When selected prevents you from moving an object.

Physical - If not selected, objects ignore the physics simulation. For unscripted objects, they will stay in one location, unaffected by gravity or collisions. If selected, objects will fall and move if hit by something else.

Temporary - If selected, the object will disappear after a short time.

Phantom - If selected, the object can be walked through or penetrated by other objects in motion.

Position - The location of the object center in map region coordinates X (east), Y (north), and Z (height) in meters. Using the number entries here rather than moving with the object axes allows more accurate positioning. The maximum values are 256 for X and Y, and 4096 for Z since that is the size of a map region.

Size - The X, Y, and Z size of an unmodified building block. Various distortions on the right side of the tab will affect the actual size. New objects are limited to 10 meters in each dimension. So-called megaprims were originally created via loopholes in the software with dimensions larger than 10 meters. If you attempt to edit the size of a megaprim, it will revert to 10 meters. In other respects they can be edited normally.

Rotation - The rotation of the object clockwise around the stated axes in degrees. X and Y are limited to -180 to +180, and Z is limited from 0 to 360.

Material - Affects how the object reacts to gravity (mass) and collision sounds. It does not affect how it looks.

Building Block Type - Lets you change the basic geometric shape.
Path Cut Begin and End - This is a vertical slice removed from the basic shape, the way a cake or pie is cut in real life. The numbers define a starting and ending angle for the cut using a scale of 1.000 for the complete circle.

Hollow - The size of a center hole in the object in percent of the full X and Y dimensions. The hole is aligned with the Z axis when the rotation is 0. Rotating the object will point the hole in a different direction. The value is limited to 0 to 95 percent.
Hollow Shape - Default will make the hole match the basic shape of the object.

Twist Begin and End - Puts twists into the object, warping its shape as well as texture alignment.

Taper - Reduces the size of the top or bottom sides (x or y axes, negative or positive) of the prim.

Top Shear - Shifts (shears/skews) the top surface of the object away from the bottom. You can shift the X and Y axes separately.

Slice Begin and End - Cuts a hole in a sphere from ring of latitude (you specify the percentage) to the top or bottom of the Z axis. The dimple cuts straight to the origin of the object (leaving a cone-shaped hole).

Features Tab

This tab applies optional features to an object.
Flexible Path - When checked, the object becomes flexible rather than rigid, and behaves according to the 8 parameters listed below. The parameters have arbitrary scales. The base of the unrotated object stays fixed while the rest moves.

Softness - Controls how 'bendy' the object is. Range is 0 to 3.

Gravity - Controls how hard gravity pulls downward on the object. Negative values pull up. Range is -10 to 10.

Drag - Controls how much viscous drag (or friction) the object appears to have when moved. Range is 0 to 10

Wind - Controls how much the object bends in response to local map region winds.

Tension - Controls how much 'force' is tugging the object away from it's base. Range is 0 to 10.

ForceX - Controls how much 'force' is applied in the X (east) direction. Range is -10 to 10.

ForceY - Controls how much 'force' is applied in the Y (north) direction. Range is -10 to 10.

ForceZ - Controls how much 'force' is applied in the Z (up) direction. Range is -10 to 10.

Light - When checked, the object emits light to the surrounding area. This is separate from glow in the Textures tab.

Color - When the box is clicked, a Color Picker floating window opens to select a color for the light.

Intensity - How bright the light is.

Radius - The distance the light extends, in meters.

Falloff - How fast the light fades from the center.

Texture Tab

Textures are the images applied to object surfaces to make them look like something besides a plain grey shape. When an object shape has loaded, but not the texture it is shown as grey.
Texture sizes are forced to be power-of-2 dimensions (64, 128, 256, etc pixels)in each direction for efficiency in graphics memory. Maximum size for a newly created texture is 1024x1024. A few legacy textures are larger.
Once textures are loaded into Second Life's database, they cannot be edited. You use any one of the many Texture Tools such as the GIMP, Photoshop, or even MS Paint to prepare a texture. The texture itself can be used multiple times and shared with other users.

Texture - Clicking this box will open the Pick: Texture floating window, allowing you to select a new texture. Alternatively you can drag a texture from your Inventory window to the box, or directly to the object. If the Select Texture item in the upper pane of the Tools window is selected, you can change one side at a time of the object. Otherwise texture changes are applied to all sides at once. If an object uses more than one texture, the word Multiple will appear in this box.

Color - Clicking this box will open the Color Picker floating window. Color tints selected are additive to the color of the base texture. This means it will be darker or more strongly tinted.

Transparency - Uploaded textures may be RGB colored, or may contain a fourth, alpha, channel which defines transparency. This setting applies an overall transparency to the texture in addition to any transparency in the alpha channel. The range is 0 to 90 percent.

Glow - This item creates a surface glow on the object, separate from the light emitted with the Features tab. The range is 0 to 1, but any glow above 0.1 tends to wash out the surface texture.

Full Bright - If checked renders the object as if fully day-lighted, regardless of the environment day cycle position of the sun.
Mapping -

Shininess - Gives the object an apparent gloss or matte appearance.

Bumpiness - Gives the object an apparent bump map shading in addition to the texture.

Repeats Per Face - Controls how many times the texture is repeated on one fact of the object. Range is 0 to 100 in each axis, and can be flipped in both axes.

Rotation - Controls rotation of the texture relative to vertical. Range is -360° to 360°.
Repeats per Meter - If a value is used here, the texture is applied according to the object size, rather than a number of times per face. Range is 0.1 to 10.

Offset - Shifts the texture horizontally and vertically relative to the object surface. If the texture is rotated, the shift is along the texture, not the object face.

Media -

Content Tab

This tab shows the internal contents of the selected object. An object can contain any type of inventory item, including other objects which in turn have nested contents. There is no set limit on the number of items that can be placed inside an object, though if it in the hundreds, it can take some time to fetch or edit the contents.

New Script - When objects are located in a map region, any active software scripts inside them are run by the host server. This allows objects to perform a wide variety of actions. Objects stored in your inventory are not actively run. This button will open a new Script window for editing.

Scripts are written in a "C++"-like "Linden Scripting Language" (LSL), which has many functions specific to Second Life. Objects may contain multiple scripts. See the LSL Portal for details. Scripts can also modify the inventory contents of prims that contain them, see Inventory for details.

Bulk change permissions Window

Permissions... - Opens the Bulk change content permissions window. This allows you to mass-edit item permissions. Note: you cannot grant yourself more permissions than you started with for the items.

The main list box - Items are listed in alphabetical order, without regard to inventory type. If an item has limited permissions, it will say (no copy), (no modify), or (no transfer) after the item name. Clicking an item will highlight it, and right-click will bring up a context menu. Editable items (notes and scripts) can be opened while inside the container. Items may be dragged from inventory to this list, or from this list to your inventory

Create Tool

This tab allows you to create a basic geometric shape (prim). The mouse pointer changes to a magic wand when this tool is active. Clicking one of the shapes in the tool, then clicking somewhere in the 3D view, will create an object at that location. New objects are 0.5 meters in size and have a plywood texture by default. The last two items, trees and grass, can only be used by the landowner. The others can be used by anyone with building permission on the land parcel.
Each land parcel has a limit to how many prims it can contain in total. If this limit has been reached, you will get an error message if you try to add another item.
Keep selected - If not checked, after creating a new object, the window will change to the edit tool so you can edit your new object. If checked the window will stay on this tool so you can make more items.
Copy selected - If checked, will create more objects of the selected type, and align them touching the face of where you click next.

Center -

Rotate -

Land Tool

This tool lets you modify the land parcel itself as opposed to objects located on the land. Selecting this tool will change your pointer to a bulldozer.
A land parcel shares part of the terrain map for the region it is located in. The top portion of this tool allows you to reshape the terrain within the Limits for the region and adjoining land parcels. The terrain map is continuous, meaning you cannot tear holes in it. So any changes you make to one part of it will smoothly connect to neighboring parts.
Estate owners can also change the texture (images) for the surface, by changing the terrain map. External tools exist to edit it. Some special plants are created internally in the viewer software, and only count for one prim each. They can be found in the Library part of your inventory. Other plants are normal objects.

Select Land - The first button is used to select an area via click-drag across the land in the 3D view. A yellow outline will appear to show your selection. The other buttons will then give you the option of applying the change to the whole selection using the Apply button, or using the Bulldozer to adjust areas as you click-hold over them. Arrows will appear over the area you are working on, and the longer you hold the mouse button, the more the land will change. The Size and Strength sliders control how effective the bulldozer is.

About Land... - Opens the About Land floating window to see or edit the parcel information.
WARNING: The following three buttons will cause loss of parcel settings in the About Land window, and may cause loss of search listings, traffic score, and landmarks may fail.

Subdivide... - Opens Subdivide Land window to break selected land area into a separate parcel.

Join... - Opens Join Land window to join parcels. Any parcels you own that have parts in the selected area will be joined. It is not necessary to select the entire parcels.

Buy Land... - Opens Buy Land window to "buy" a land parcel. See the discussion there about how this is not an actual purchase, but payment for services.

Abandon Land... Opens Abandon Land window to return land to the region owner (Linden Lab or Estate owner). Note it is often better to sell than abandon a land parcel, since you get no payment when you abandon one.

Source:
http://wiki.secondlife.com/wiki/Building_Tools
http://community.secondlife.com/t5/English-Knowledge-Base/Build-Tools/ta-p/700039
http://wiki.secondlife.com/wiki/Prim

Extra Links
http://www.shiny-life.com/2007/10/21/top10-second-life-building-tips/
http://www.mermaiddiaries.com/2006/11/build.html
http://community.secondlife.com/t5/English-Knowledge-Base/Building-tips/ta-p/700041