Basic Prim Creation and Editing
From Second Life Wiki
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".