X-Windows provides computers with a graphical user interface. The difference to most graphical user interfaces is, that any program displayed can be run on any other computer, that the displaying computer is connected to. X-Windows is completely separate from the operating system, which means programs can do their work and displaying separately. This is a major difference to graphical user interfaces like MS-Windows, where the working of the program is heavily dependent on the displaying of information.
X-Windows is available on a multitude of computers. There are versions for UNIX, VMS, MS-Windows, the Apple Macintosh and many other platforms. For UNIX and VMS they are the primary graphical user interfaces.