There are a number of X Windows variants. X Windows is an industry standard way for software to send text and pictures to a user's screen and accept inputs from a mouse and a keyboard. As you can imagine the interface between the terminal and the software is far more complex than that required simply to write text sequentially to the screen and take input from a keyboard. The X Windows standard ensures that, so long as the software ``talks'' X and the terminal can run X, the software should run properly on the computer. As it is possible to ``talk'' X over a network it is quite normal to have the software running on one computer and have the user interact with it on another. Indeed a user might open up one window on his or her terminal for local work and then log into one or more remote computers and have them display one or more windows on the terminal as well. Linux, the version of UNIX we use in the department, offers a number of GUIs based on X Windows, for example Gnome and KDE.