The X Window System is a distributed, graphical method of working developed primarily at the Massachusetts Institute of Technology . It has since been passed to a consortium of vendors (aptly named ``The X Consortium'') and is being maintained by them.
The X Window System (hereafter abbreviated as ``X'') has new versions every few years, called releases. As of this writing, the latest revision is X11R6, or release six. The eleven in X11 is officially the version number.
There are two terms when dealing with X that you should be familiar. The clientclient is a X program. For instance, xterm is the client that displays your shell when you log on. The serverserver is a program that provides services to the client program. For instance, the server draws the window for xterm and communicates with the user.
Since the client and the server are two seperate programs, it is possible to run the client and the server on two physically seperate machines. This is the real beauty of X. In addition to supplying a standard method of doing graphics, you can run a program on a remote machine (across the country, if you like!) and have it display on the workstation right in front of you.
A third term you should be familiar with is the window managerwindow manager. The window manager is a special client that tells the server where to position various windows and provides a way for the user to move these windows around. The server, by itself, does nothing for the user. It is merely there to provide a buffer between the user and the client.