It's difficult to describe the X Window System in a nutshell. X is a complete windowing graphics interface that runs on almost all computer systems, but was established mostly on Unix and now on Linux. X provides a huge number of options to both the programmer and the user. For instance, at least half a dozen window managers are available for X, each one offering a different interface for manipulating windows. Your distribution has chosen a window manager along with a desktop. By customizing the attributes of the window manager, you have complete control over how windows are placed on the screen, the colors and borders used to decorate them, and so forth.
X was originally developed by Project Athena at MIT, by MIT, Digital Equipment Corporation (DEC), and IBM. The version of X current as of the time of writing is Version 11 Revision 6 (X11R6 ), which was first released in April 1994 and then subsequentially updated in minor versions. Since the release of Version 11, X has virtually taken over as the de facto standard for Unix graphical environments.
Despite its commercial use, the X Window System remains distributable under a liberal license from the Open Group. As such, a complete implementation of X is freely available for Linux systems. X.org , the version most directly based on the X sources, is the version that Linux uses most often. Today, this version supports not only Intel-based systems, but also Alpha AXP, MicroSPARC, PowerPC, and other architectures. Further architectures will follow. Support for innumerable graphics boards and many other operating systems (including Linux) has been addedand X.org implements the latest version, X11R6.8.2.[*]
We should mention here that commercial X Window System servers are available for Linux that may have advantages over the stock X.org version (such as support for certain video cards). Most people use the X.org version happily, though, so this should certainly be your first stop.
As we mentioned in "Why Use a Graphical Desktop?" in Chapter 3, people who run Linux as a server often don't install X at all. They control the server through remote access only, or using just the text interface.