Primitive Type - Shape
Creates a primitive shape at the size you choose. The choice of shapes is:
Sphere
Cube
Cylinder
Torus
Cone
Plane
This shape can be textured like any other object, but only exists as a virtual object purely in Layout and saved in the scene file. It can even be used for Bullet.
Radius - determines how large the object will be initially and acts as a multiplier for scale values applied to the null.
Displacement Height - Controls how far out from the surface a displacement (set in the Node Editor Surface node).
Accuracy - Distance multiplier control to deal with primitive edge cases that need smaller marching steps.
Object UV - All Primitive shapes have an inherent UV map called Object UV. This can be used as a mapping source in Image nodes. The Image Sampler node doesn't need a Projection to use Object UV, loading the image and attaching it to the input you want on your shape is enough.
Shape Pros and Cons
Don't consume memory like geometry
Resolution-independent
Can be surfaced
Can be used for Bullet Dynamics
Are saved in the scene file
Object UV
Only the simple preset shapes supplied (third parties can write others)
You can't add FiberFX to primitive shapes
Cannot be used for Instancing
Slower to render if displaced
Since they are based on nulls, most "Object-level" operations won't work (such as the Object ID buffer)
Example - Shape Dynamics
Example - Displacing Primitives