[Mark Reis]

Clear Unused Regions removes (or deletes, depending on your selection) regions from the region bin that do not appear in a track.

Consolidate concatenates the highlighted regions, utterly destroying the data that is in the source file for those regions, but outside the data actually in the selected regions.

In simpler terms, Consolidate says "Throw out everything else in this file ( or these files) except for these regions and shorten the file accordingly, discarding all unselected data."