It's important to understand the difference between shapes and bodies. A Body has a position and orientation in the World cooridnate system, whereas shapes are attatched to a Body. Shapes simply act as the collision geometry of the Body. Why bother differentiating betwen the two? Because it allows us to create advanced Bodies composed of many shapes.

It is worth noting that shapes collide, but Bodies don't. This means that even if you attatch several shapes to a single Body (for instance a gradually destructible "boss"), you'll easily be able to detect which parts collide with what.