XOrg is the XServer software that most Linux and BSD distributions use. It is the reference X11 X server from the "X.Org Foundation", the consortium responsible for setting X11 specifications. It forked from XFree86 after the developers decided to relicense.
The XServer code is released under a Free license allowing anyone to make their own proprietary modifications; most commercial X servers are based on the XOrg reference X server.
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