The DesktopMetaphor is a HumanComputerInteraction realized as a virtual 'desktop' upon which are scattered 'files' that one works upon one at a time - often working on the 'topmost' file, which overlaps other files on the desktop. When files aren't in use - 'open' and 'on the desk' - they are inactive entities. Files can be organized into 'folders' and tucked away into strange hierarchies never to be seen again, at least not without a good memory or the computer-equivalent of the card catalog - a good index.
The DesktopMetaphor is so pervasive today that most people outside OR inside the computing industry have no clue that there are real alternatives to it. But see DontModeMeIn and NoApplication. But the mold is slowly being chipped away... with ServiceOrientedArchitecture, TheNakedObjectsFramework, AJAX applications, PublishAndSubscribe architectures, RSS, and innovative projects like SeaDragon? and QedWiki that combine individual 'files' into something much greater, GnuTella and Ocean other forms of 'flat' distributed data storage with indexing & search, and mobile computing in the form of advanced cell-phones with services that users value (like the ability to listen to music and talk to one's friends and conduct business and get directions to the nearest four-star Indian food joint...)
Someday, I hope I'll have the pleasure to say this of the DesktopMetaphor: 'Good riddance'. The DesktopMetaphor tends to tie one to a keyboard and terminal, and tends to isolate people to their own little cubby-holes where they can't talk to anyone without double-clicking on some equivalent of the desktop's 'phone' (IRC or e-mail). What I want is PervasiveComputing and communication where interaction and interdependence is the default, and isolation comes from building walls.