The GNU Free Documentation License, or GNU FDL, is an intellectual property license specifically designed for written material that is intended to be freely available. It is the counterpart of the GNU GPL, which provides a similar license for software.
The license requires, in general, that those who copy the licensed material must in turn make that material freely available under the same license and must also provide a copy of that license along with the material. (In the case of web-hosted material, a link to a web page containing the license is sufficient.)
Because most IP licenses are intended to protect the owner's copyright by preventing copying, the GNU FDL (which encourages copying) is often referred to as a "copyleft" license.