Well, term "Workspace" was used by earlier versions as top-level menu for all operations involving project. It is replaced by more logical "Project" now.
.var files represent assemblies (the ordered set of nest directories)
.cfg files are used to store any current user options, information etc... There are "project" related .cfg files (means some options are stored on per-project basis; there is also one .cfg for global settings.
.usc files contain "scripted metainformation" of package. The most important content is description of widgets, including "Paint" method - this is then used by the Layout designer.