The GNU General Public License (GNU GPL or simply GPL) is a widely used free software license that guarantees end users the four essential freedoms: to run, study, share, and modify the software. It was created by Richard Stallman to protect GNU software from being made proprietary and is a specific implementation of the "copyleft" concept, which ensures that software remains free. The GPL is a copyleft license, meaning that any derivative work must be distributed under the same or equivalent license terms. It is more restrictive than the Lesser General Public License and distinct from permissive software licenses such as BSD, MIT, and Apache. The GPL requires that the terms and conditions under which the work is distributed be made available to everyone who obtains a copy of it with a GPL applied to it. The license is designed to protect users rights and ensure that software remains free for all users.

In summary, the GNU General Public License is a crucial tool in the world of free and open-source software, providing a legal framework that ensures software remains free and accessible to all users.