Primitives are single convex brushes within Hammer. This includes blocks, wedges, cones, cylinders, and even spheres. Arches and toruses are not primitives but groups of primitives.
Blocks:
File:Cube.gif
Wedges:
File:Wedge.gif
Cylinders:
File:Cylinder.gif
Spheres:
File:Sphere.gif
Arches:
File:Arch.gif
Toruses:
File:Torus.gif
