The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for free content, designed by the Free Software Foundation (FSF) for the GNU project. It is the open content counterpart to the GNU GPL. The current state of the license is version 1.2, the official text of which can be found at http://www.gnu.org/copyleft/fdl.html.
The license was designed for manuals, textbooks, other reference and instructional materials, and documentation which often accompanies GPL software. However, it can be used for any text-based work, regardless of subject matter. It stipulates that any copy of the material, even if modified, carry the same license. Those copies may be sold but, if produced in quantity, have to be made available in a format which facilitates further editing.
Projects under the GFDL include Wikipedia, which is the largest documentation project among them. The GFDL is also used almost exclusively on Wikicities (including this site). Uncyclopedia and Memory Alpha are considered sister projects of Wikicities, rather than part of Wikicities, and use Creative Commons licenses instead of the GFDL.