Part and Part Design
There has been much discussion of this over the years about the differences and ramifications of using the Part and Part Design workbenches.
It is a good idea to use one or the other until the user is comfortable with one, then learn the other. It is also typically recommended that new users not mix them until the ramifications are of doing so are understood.
Let's talk about those ramifications.
Part Workbench Concepts
It is often said Part WB is more CSG style modeling. This is where the operator combines various primitives to end up with a lump in the desired shape. (In fact Part WB goes one step further than just primitives and allows the operator to use a sketch+extrude operation to create random shapes as well.) When each primitive or shape is created, it has no relationship to other objects created, it is a single solitary solid.
This condition remains so, until, the operator uses some operation to combine them (typically a Boolean that adds or subtracts them).
The take away is the single solitary solid bit and the combining them bit.
Part Design Workbench Concepts
In Part Design WB the Body object represents a single solitary cumulative solid. The first lump created under the Body (be it a Pad from a sketch, or a BaseFeature from outside the Body) represents a lump of raw material that will be further processed to refine it to the desired final shape (solid). It is cumulative in the sense that each operation adds or removes material. The lowest/last operation (the current Tip of the Body) is the current state. Any other feature under the Body but above the Tip, does not represent a complete/stand alone solid. Only the Body (basically a proxy for the tip) or the Tip represent a complete solid.
This image shows a Body. It is a cumulative solid that consists of a padded sketch and a cone primitive. This is a single solid. Neither the pad or the cone can exist separately.
(Another thing mentioned often is a Body MUST be a single contiguous solid. This means all geometry created by a feature in the Body must touch it's predecessor.)
People get caught when they attempt to use some feature under the Body (rather than the Body itself) as one selection of a Part WB Boolean operation. This is a problem, because the selected feature does not represent a complete solid.
In a sense, from a Part WB standpoint, the Body represents another primitive. So, using a Body (remember it is a proxy for the tip) and a Part WB object to do a Boolean is valid. But the resulting object is a Part WB object. And, thus Part Design WB tools can't be used on it any longer.
And, it can get even more complicated. If you create a new Body and drag the result from the previous paragraph into it, a BaseObject is created. And you can go off an use the Part Design WB tools on it.
There is a caveat with the Tip and it's representation of the single solid in the Body. If the tip is a subtractive feature and is used in a dress up operation, for instance a Mirror, the Mirror is operating on the underlying feature (a pocket for example). Thus the cumulative solid is not mirrored, but the subtractive feature is. The result of this must create a single solid.
In this example, a mirror of the tip (which is the pocket of the slot) around any of the base planes, or even a face of the solid will not produce a mirrored solid of the entire model. (In fact, it will produce a Mirrored feature in the tree that is essentially empty.)
In this example, a mirror of the tip (which is the pocket of the slot) is performed around the datum plane and produces a mirrored slot:
For more information on the Part Design Mirror dress up tool: Part Design Mirrored
Part and Part design workbenches can be used together with some care, creating quite complex models.