From xfree86.org:

The XFree86 Project, Inc is the organisation which produces XFree86?, a freely redistributable open-source implementation of the X Window System.
You might want to check out the X home page: http://www.x.org/

Introduction: About the X Window System
The X Window System, more simply 'X' or 'X11', is judged worldwide to be one of the most successful open source, collaborative technologies developed to date. It is the de facto standard graphical engine for the UNIX* and Linux* operating systems and provides the only common windowing environment bridging the heterogeneous platforms in today's enterprise computing.
HTH