Primitive
Primitives are single convex brushes within Hammer. They include arches, blocks, cylinders, spheres, spikes, tori and wedges.
Primitive types

Arch
Must be created from the top view and rotated into whatever position you want it in.
Block
The block, a basic cube, is the most used object when mapping in Hammer.
Cylinder
Cylinders are second in popularity only to blocks. They must be created from the top view and rotated into position.
Sphere
The sphere is a rather complicated piece of geometry, so it would be wise to be careful with the number of faces you give the sphere.
Spike
Torus
The torus resembles an inner tube or a donut. This is most useful for rings. This primitive can easily mass polygons so keep the number of faces on this primitive low.
Wedge
The wedge is best used for creating ramps. It is basically a triangular prism.