Commonly referred to as a "Desktop" by modern WindowManagers. Its a single instance of a collection of windows, of which there may be many on a machine. They are used quite successfully to alleviate the problem of OverlappingWindows? (a major HCI headache in my opinion -- DanielLawson), by allowing users to dedicate a whole ViewPort to a single application. EG: One for web browsing, one for email, one for XMMS, one for coding, one for FreeCiv and so on.