X Window System
The X Window System is a popular user interface technology used to build Windowed applications and services, most commonly on UNIX computers. It is governed by the X11 ANSI standard (see the X11 definition for more information), and administered by the X Consortium.
The X Consortium is yet another sterling effort of the MIT Laboratory for Computer Science (the parent group in the U.S. for the W3C), and the group whose responsibility is to "develop, evolve and maintain the X Window System, a vendor-neutral, system-architecture-neutral, network-transparent windowing and user interface standard" according to their own materials (see URL below). In English, this means that the X Window System is a platform-independent, graphical, windows-based user interface that is described by a formal industry standard. In practice, X Windows (as the X Window System is sometimes called) is most often used in the UNIX environment, where it provides an excellent way to unite the many flavors of UNIX into under a more coherent, understandable user interface. Other implementations abound (for DOS, Windows, Macintosh, etc.) but are not as commonly used as in the UNIX community.