The version of the X Window System most commonly used on GNU/Linux operating systems.

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.

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