The X window system is a client-server based GUI environment. It was first developed as part of the Athena project at the Massachusetts Institute of Technology with the help of DEC. First released in 1984.
The version most commonly used nowadays is XFree86 which is based on X386.
Now that powerful processors and video cards are common, the client-server functionality of the X Window System is starting to fade. It's still implemented, but the main use now is on desktop computers.