Virtual Collections are really pre-defined searches, and don't actually hold any data of their own. For example, the "Today" collection is a query that searches for clips captured today. The "Bitmaps" collection is a query that searches for all clips containing Bitmaps.
ClipMate 7 ships with several pre-defined virtual collections. You can make your own, if you understand SQL. Create the new collection under "Virtual", and set the properties so that it's a virtual collection. Then enter some SQL. You can use the others as examples. If you delete all of the virtual collections, they will be re-built at startup.
There are several special-case virtual collections: