The GNU Free Documentation License (GFDL) is a copyleft license for open content, designed by the Free Software Foundation (FSF) for the GNU project. The official text of version 1.1 of the license text can be found at http://www.gnu.org/copyleft/fdl.html. Here is Wikipedia's uneditable copy of Version 1.1 of the license; this needs some more work before it constitutes a proper copy of the license.
The license is designed for software documentation and other reference and instructional materials. 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.
The license explicitly separates the "Document" from "Secondary Sections", which may not be integrated with the Document, but exist as front-matter materials or appendices. Secondary sections can contain information regarding the author's or publisher's relationship to the subject matter, but not any subject matter itself. While the Document itself is wholly editable, and is essentially covered by a license equivalent to the GNU GPL, some of the secondary sections have various restrictions designed primarily to deal with proper attribution to previous authors.
Specifically, the authors of prior versions have to be acknowledged and certain "invariant sections" specified by the original author and dealing with his or her relationship to the subject matter may not be changed. If the material is modified, its title has to be changed (unless the prior authors give permission to retain the title). The license also has provisions for the handling of front-cover and back-cover texts of books, as well as for "History", "Acknowledgements", "Dedications" and "Endorsements" sections.
Using the GFDL
For a document to be covered by the GFDL, one must include a specific copyright and license notice.
Note about Wikipedia's use of the GNU Free Documentation License
All Wikipedia (and Nupedia) content is covered by the GNU Free Documentation License. The details necessary for proper licensing under the GFDL are being discussed right now on Wikipedia-L (see also /Workshop).
Wikipedia and Nupedia's use of the GFDL began in January, 2001, and has won the project the support of Richard Stallman of the FSF. See . It has long been the understanding of Wikipedia principals (Jimbo Wales and Larry Sanger at least) that, as in the case of Nupedia (see  and ), links back to original Wikipedia articles would be required from anyone who used Wikipedia articles. (Jimbo confirms that Stallman agreed that the license permits this. ) Wikipedia principals have, recently, finally gotten around to making this requirement explicit for Wikipedia (as it has been for Nupedia), which has caused some controversy. (See the Wikipedia-L archives and /Talk.)
One of the issues being discussed is the requirement (by Wikipedia principals Jimbo Wales and Larry M. Sanger in October 2001) that one use a HTML banner (back reference) on any page on another web site or CDROM on which Wikipedia content appears, or (also now strongly supported by Wikipedia principals) a text link. Please note the GNU Free Documentation License (GFDL) says that the license must be included in every copy of a document. This means that the back reference should at least include a link to the GFDL which is not the case yet in the Wales/Sanger proposal (or requirement as they like to put it).
Note that international copyright law (including the US law) states they cannot change the license without the written consent of the other authors. The principals are aware of this but they believe their requirement/proposal falls within the scope of the license.
Please see Wikipedia's license instructions page for details. What exactly can and should be required is under active consideration, and the founders and managers of the Wikipedia project will be working together with all interested Wikipedians to arrive at a mutually agreeable solution on Wikipedia-L.