The GNU General Public License is the copyleft license that accompanies GNU software wherever it is up or downloaded and incorporated into creative projects. GNU (stands for GNU’s Not Unix) is a freeware operating system compatible with UNIX introduced in the early 1980s in response to the frighteningly monolithic dominance that proprietary operating systems had over computing markets. “Free” does not refer to price, as explained by the Free Software Foundation, but rather to liberty, i.e. more akin to the ‘free’ in ‘freedom of speech’ than in ‘free beer’:
You may or may not pay a price to get GNU software. Either way, once you have the software you have three specific freedoms in using it. First, the freedom to copy the program and give it away to your friends and co-workers; second, the freedom to change the program as you wish, by having full access to source code; third, the freedom to distribute an improved version and thus help build the community. (If you redistribute GNU software, you may charge a fee for the physical act of transferring a copy, or you may give away copies.)
The Free Software Foundation is the primary sponsor of the GNU Project.