On this page you can find information aboutthe GNU General Public License, probably the most important license for free software.
The GNU General Public License is the archetype of the copyleft licenses. The operating system GNU/Linux and countless other programs made it quite influential and various other open source licenses are based on the GPL.
In this area you can find commentary on the GPL v2 in the form of a book that was published in 1991. Furthermore, an overview is given over the various changes in GPL v3 that was finalized in 2007. The changes mainly are in response to technical developments.