Workspaces & Profiles. Can't put it any better than dei-feif who posted this reply to a similar question at the AutoDesk Discussion Groups on 11-Nov-06:
"A Profile is a set of various user settings, some of which include the current settings for the Main and Enterprise CUI files.
Workspaces are screen layouts, and contain an arrangement of toolbars, menus, and palettes. Workspaces are stored inside of CUI files."
Does that help?