The GNU General Public License, often shortened to GNU GPL (or simply GPL), lists terms and conditions for copying, modifying and distributing free software. The GPL was created by Richard Stallman in order to protect GNU software from being made proprietary. It is a specific implementation of his "copyleft" concept. According to Stallman, copyleft is a derivative of copyright law that serves "the opposite of its usual purpose: instead of a means of privatizing software, it becomes a means of keeping software free." (The name was taken from the humorous slogan, "Copyleft -- all rights reversed.")
The first term of the GPL states that programmers may freely copy and distribute verbatim copies of a software program's source code, under the condition that each copy displays a copyright notice, disclaimer of warranty, intact GPL notices, and a copy of the GPL. The second term of the GPL, which deals with modification and redistribution of software, requires the same conditions as the first, along with notification of any changes made to the software.
The GPL itself is under the copyright ownership of the Free Software Foundation (FSF), a tax-exempt charity organization founded by Stallman's GNU project in order to generate funding for free software development.