The X Window System comes in both proprietary and open source forms. There is a great deal of development in this area, and as more development happens, the better (and easier) working with Unix will become. But don't fear—the current versions of the X Window System happen to be very good and fairly user friendly if you are already familiar with some form of GUI, such as Microsoft Windows.
The X Window System is simply referred to as X. There are also revisions of X, so you will commonly find a number following the X when you see it. Here is an example of the current version of X:
Right now, X is at X11, or its 11th major revision. What about the R6? What does that mean? Well, that's easy to explain. The ...