Cheetah 3D gives you parametric primitives; the cube, sphere, torus, etc. These primitives, as Helmut said, have their own properties which can be changed at will. The ability to change the size, angle, etc. is what makes them Parametric. The primitives themselves are calculated by the software using math formulas. They are quick to load, very light on memory requirements, and can be changed when the user decides to alter their properties. The software changes some variables in the formula and recalculates the primitive. This is very similar to the way that vector shapes are calculated in Illustration programs.
To begin editing those primitives however, you can no longer use the math formulas that create them. Instead, they are converted into polygon faces, edges, and points. Now that they are Editable, you can begin shaping them to your liking. But a sphere is most likely not a sphere anymore. It is a polygon form that you begin moving and shaping into something like a character. This is similar to the way Bit Mapped graphics are created and edited in a Photo Editing program.
Both types of geometry have their uses. Hope that helps.