GNU GPL (GNU General Public License) is a free software license, perhaps the most widely used today. GNU is a project to advance the creation and use of free software. The GPL is a “copyleft” license that requires derivative works be distributed no more restrictively than the GPL itself.
GNU (which stands for GNU’s Not Unix) was started by Richard Stallman in 1983 for the advancement of free software. Its crowning achievement is Linux, a GNU GPL operating system based on Unix. The GPL is the standard version of the “copyleft” license, required of all software developed or distributed under the GNU umbrella.
“Copyleft” is a term generally applied to the use of copyright law as a means of removing restrictions on the use of the works of authors, who otherwise would be legally entitled, and sometimes ever required, to restrict the distribution of their materials.
Distinguished from the GPL is the BSD license (or MIT License), which are permissive, free licenses without the restriction on the licensing of derivative works. They are close to being dedications to the public domain, but are not complete renunciation of authors’ rights.