What is the X in X-mas?
(GPL, note US spelling) The licence applied to most software from the Free Software Foundation and the GNU project and other authors who choose to use it.
The licences for most software are designed to prevent users from sharing or changing it. By contrast, the GNU General Public License is intended to guarantee the freedom to share and change free software - to make sure the software is free for all its users. The GPL is designed to make sure that anyone can distribute copies of free software (and charge for this service if they wish); that they receive source code or can get it if they want; that they can change the software or use pieces of it in new free programs; and that they know they can do these things. The GPL forbids anyone to deny others these rights or to ask them to surrender the rights. These restrictions translate to certain responsibilities for those who distribute copies of the software or modify it.
See also General Public Virus.