"Expand" flattens the illustration, or creates lower-level sub-objects from illustrator live effects. For example, it turns a path into outlines or any applied effects such as a 3D revolved object into a static object with individual paths that you can edit. The original effects are removed and cannot be edited in the way they were prior to expanding.
"Expand Appearance" does the same, but does it for those modifications that have been applied in the Appearance panel.
So it is basically the same command for effects created in two different ways, and the command affects only those effects that are either Appearance attributes or those that aren't.