Primitives in Second Life are constructed by starting with a basic two dimensional shape, modifying it, and extruding it in to the third dimension along a path while applying modifications to the extrusion. The most basic example is a cube which starts out as a 2D square and extrudes along a linear path with no modifications to make a cube. A cylinder starts out as as circle and extrudes along the same linear path. If the circle was extruded along a circular path you would create a ring, and if a half-circle is extruded along another type of circular path you create a sphere.
2D Shape (Profile)
2D Shape (Profile) Modifications
Extrusion (Path) Modifications