Whenever you talk about a GUI under Unix or Linux, your first thought is KDE or Gnome. But KDE and Gnome would be nothing without the X Window System, or more simply X11 or X, on which they rely. Free Software Magazine has an excellent, in-depth look to answer the question What is X?.

X is a windowing environment that provides a bare skeleton for GUI operations. Ignoring the display elements for a second, one of the key elements of X is that the computer on which the application is running, and the machine on which the interface to that application is provided do not need to be the same machine. I can run Mozilla on one of my Linux servers, but control and use the application on my Mac OS X desktop.