With the arrival of graphical UIs on desktop computers, Unix workstation vendors saw a need for provide an interoperable standard for running a graphic applications on Unix.
In 1984, MIT delivered the X Window System, which not only provided a graphic user interface, but also delivered network transparency. That meant an X Window client application could run on one system, and present its graphical interface to the X Window server running on another machine.
X Window is also used to deploy thin clients. Unlike Microsoft's Terminal Services however, users sessions in Unix can efficiently share access to various system libraries. As noted above, in Windows NT every concurrent user has to maintain an entire Win32 subsystem solely dedicated for their own use.
The X Window serves mainly to abstract the video display hardware so that Unix software can run on a variety of vendor's hardware. X Window is a lowest common denominator for getting graphics applications to run. It's not pretty, consistent, or functionally complete.
The specification doesn't define a window manager or a desktop programming environment. The resulting variety of possible combinations of window managers, GUI options, themes, widgets and development environments associated with X Window are reminiscent of the phrase "the best things about standards is that there are so many to choose from!"
In the late 80's, Sun offered a complete window system called NeWS, which was based upon PostScript. NeWS included an object oriented programming environment, so that entire applications could be built in PostScript code.
While NeWS offered far more functionality, Sun's licensing fees and the threat of Sun taking over as the standards overlord of the Unix world resulted in other vendors lending their support to X Window.
Around the same time, Adobe and NeXT teamed up to deliver an improved display system called Display PostScript. Like Sun's NeWS, it was based upon PostScript, but it was solely an imaging model and language, and did not include a window system or application development layer.
IBM and SGI paired Display PostScript with X Window, but NeXT built a custom window engine for DPS that closely integrated the PostScript drawing system with their object oriented development environment.
Applications running on NeXT's version of DPS could also stream their PostScript display code to a window server on another NeXT machine using the NXHost command. This provided NeXT a remote display feature akin to X Window.
Mac OS X Graphics