The GNU General Public License (GPL) has been released by the Free Software Foundation and is a license for open source software. GPL gives more freedom than other licenses. For example you are allowed to use a program without any restrictions for all purposes, comercial purposes as well. Copies of the software can be spreaded for free. License fees are interdicted. The program can be customized to the particular needs.

Due to the so called copyleft-principle you can spread customized and changed programs only if you are doing this under the GPL as well. The GPL has been written by the founder of the GNU-Project Richard Stallman. Since the release of version 2 in 1991 there was no revision anymore. But in the moment the work for GPL 3.0 is in progress.

Further Information about GPL on Wikipedia.