The XFree86 Project, Inc is the organisation which produces XFree86™,
a freely redistributable open-source implementation of the X Window System.
XFree86 runs primarily on UNIX® and UNIX-like operating systems such as Linux,
all of the BSD variants, Sun Solaris x86, Mac OS X (via Darwin), as well as other
platforms like OS/2 and Cygwin.
XFree86, provides a client/server interface between display hardware (the mouse,
keyboard, and video displays) and the desktop environment while also providing
both the windowing infrastructure and a standardized application interface (API).
XFree86 is platform-independent, network-transparent and extensible.
In short, XFree86 is an open source X11-based desktop infrastructure.
Taken directly from the www.xfree86.org