GNU General Public License
The GNU General Public License (GPL) is a computer software copyleft license. This license lets the user of the software use a program in many of the same ways as if it were public domain. They can use it, change it, and copy it. They can also sell or give away copies of the program with or without any changes they made to it. The license lets them do this as long as they agree to follow the terms of the license. The GPL was created by Richard Stallman. The current version is version 3, created in 2007, although some software still uses version 2, created in 1991.