Joomla! is distributed under the GNU General Public License version 2. A copy of that license should be distributed with every copy of Joomla! or derivatives of Joomla!. For complete information about the GNU GPL please visit the Free Software Foundation.
The GNU General Public License Version 2 or later. http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
Within the context of this FAQ, "GNU GPL" and "GPL" refer to the GNU General Public License Version 2.
Joomla!'s Copyright Policy
The Joomla! project does not aggregate the copyrights of its code contributors. No contributor is required to transfer his/her copyright to Joomla! and each individual contribution will forever remain in the ownership of its author. This also applies to translations of the Installation language set as these alone are included in the core release of Joomla!
We believe that the best license for Joomla! extension developers to use is the GNU General Public License version 2. The instructions below explain how to apply the GNU GPL to your program. The process is easy and does not require you to have a lawyer and provides you with solid protection of your work. These instructions are excerpted from the GNU GPL version 2 document at Gnu.org. If you have any questions about licensing you extension using the GPL, please contact the Free Software Foundation.