“X Window, X11 or ‘X,’ as it is known for short, provides the
programming framework and the underlying runtime system for most
Unix and Linux-based network-transparent windowing implementations.
It runs on a huge number of Linux and Unix flavors, including Mac
OS X and with a bit of help, on several Windows varieties. Without
X11 , there is no KDE, no GNOME, and no Linux-based window manager,
unless one is prepared to accept an X replacement. They do exist,
and many carry a proprietary license, while X comes with a
GPL-compatible license.“Many Linux and Unix desktops and window managers should
actually be called X Desktops, since they use the X Window
framework to provide users with a full, bitmapped system GUI.
Still, most Linux and Unix users would not actually see X Window
directly, except when they run X applications, like the xterm and
rxvt terminals or some fairly basic games. On its own, X Window
requires, but does not provide software to manage and display
flexible GUI elements, e.g., windows. It provides the primitives to
display them, however, including the ability to draw graphical
elements and, of course, strings of characters…”