The version of the X Window System most commonly used on GNU/Linux operating systems, including Gnome? and KDE desktop environments.

The current version is 11, and the current release of this version is 6, so you might see things like X11R6.

From the X(1) manual page

The X Consortium requests that the following names be used

when referring to this software

X

X Window System

X Version 11

X Window System, Version 11

X11

However, many people still refer to the system anyway as "XWindows".

X is responsible for drawing graphics (including text). Without X, you would only have the text mode that is build in to most display adapters (eg what you see when linux is booting up). XFree86 is a FreeSoftware implementation of the X protocol originally designed for the free x86 unix operating systems but now supports non-x86 hardware.

XFree86 version 4.3 (released Feb/Mar 2003) has lots of cool features. Some of the notable ones are:

• ability to configure which keys are used for changing virtual terminals (used to be hard-coded to ctrl+alt+ (F1 - F6) and which keys are used for changing screen resolutions (used to be hard-coded to ctrl+alt+ (KP_+ and KP_-).
• The Cygwin port can now run "rootless". This means that you can use Microsoft Windows as your window manager, whereas previously cygwin needed one window and ran all x applications within that single window. This was the biggest advantage that commercial win32 xservers had over the free ones, until now :)
• AddToMe