Consider a steering wheel. The main parts are the hub, the rim, and the spokes which join those two.
If the hub and rim are created separately (not merged) from each other, they would be bodies within a part. That is known as a multi-body part.
The hub & rim would be created using several features. (extrudes, revolves, holes, etc).
When the spokes are created (using features), they can be left not merged with the hub & rib, in which case they would also be separate bodies within a part. If the ribs are merged with the hub & rim, then all those bodies would become one body within the part.
So, features create bodies, bodies create parts.