GNU Free Documentation License
The GNU Free Documentation License (GFDL) is a copyleft license for free content, designed by the Free Software Foundation (FSF). Please read the text of the GNU Free Documentation License for full details of this licence.
The GFDL is a copyleft license, meaning the content can be copied, modified, and redistributed so long as the new version grants the same freedoms to others and acknowledges the authors of the content. Such licensed content will therefore remain free forever and can be used by anybody subject to certain restrictions, most of which serve to ensure that freedom.