A client/server system originally developed at MIT in the 1980s to allow a workstation, running under UNIX and equipped with keyboard, screen, and mouse, to support an interactive graphics environment.

A window is a rectangular area on the display screen whose position, size, and contents can be controlled by an application. There may be many applications active at any one time, and each application may have more than one window active at any one time. Windows are created, managed, and closed down by requests from the applications or clients, to a server, the windows manager.

The definition of the X Windows system covers both the behaviour of the windows manager, and the form and content of messages that pass between the windows manager and the client applications. The windows manager and the client applications may either coexist on a single workstation (typically a small UNIX system), or some of the client applications may reside on other systems connected by a network to the workstation that runs the windows manager. Somewhat confusingly, some of the client applications may well run on a server; for example an application requiring the completion of an extensive arithmetic calculation.

http://www.x.org/wiki/ The X.Org Foundation home page