XOrg is the reference X11 implementation from the "X.Org Foundation," the consortium responsible for specifying X11. It was Forked from XFree86 after that project's developers decided to relicense, and has since become the XServer software that most Linux and BSD distributions use. XOrg is released under a Free license allowing anyone to make their own proprietary modifications. Most commercial X servers are based on it.

See Introduction: About the X.Org Foundation