The GNU General Public License is a free, copyleft open source license for software and other kinds of works. This is in contrast to permissive licenses like BSD, which impose minimal restrictions on the redistribution of covered software. The GPL ensures that granted freedoms are preserved whenever the work is distributed, even when the work is changed or added to.
See the Wikipedia article on the GPL for more information.
GNU General Public License v3
Version 3 of the GPL is the latest version, written in 2007. It is the recommended version of the GPL to use for new projects.
The license text can be found at:
GNU General Public License v2
Version 2 of the GPL is an older version from 1991, which lacks some language relating to patents, as well as prevention of Tivoization. It is recommended to use GPLv3 instead if possible.
The GPLv2 license text can be found at: