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.

Viewports can also be larger than your physical display, although this feature is rarely used these days. For example, you could have a 1600x1200 ViewPort on an 800x600 display, which can very easily lead to confusion for people who don't know your setup.