GNU General Public License

The GNU General Public Licenses (GNU GPL or simply GPL) are a series of widely used free software licenses. The GPL is a copyleft license, which means that it guarantees end users the freedom to run, study, share, or modify the software, but if you distribute a derivative work or modification, you must provide the source code to those recipients under the same or equivalent license terms — there is no requirement to publish anything to the public at large.

Source: Wikipedia — GNU General Public License (CC BY-SA 4.0)