The parts library is a set of mechanical parts implemented in OOML. The difference with Primitive Objects lays on the fact that parts are composed by the addition, difference, etc of primitives or other parts. Conceptually Primitive Parts have a geometry entity, but not a real entity, and Sphere, for example, is a geometry object, which does not really exists in the world. A wheel, on the other side (even if modeled using a Cylinder) is a real existing thing, and thus, a Part.
The OOML criteria is: