X,known as X Window is the protocol used for displaying graphics and event management (mouse, keyboard, etc.).. This software is designed to operate through a network (X provides only basic functions and does not display buttons, to decorate their windows or allow manipulation: It is the task of layers on Windows manager(Metacity, KWin, ...), IceWM desktop environment (Gnome, KDE ...) and graphics libraries (GTK, Qt ..)).
The most common protocol is version 11, where X11.
There are several software that implements the X11 protocol through which the most famous is XFree86.
Due to internal conflicts and issues licenses, the project has X.org fork from source XFree86 and now follows its own road.
Apart from XFree86 and X.org, there are also other implementation of the X11 protocol, as well as commercial free.