GNU Free Documentation License

The purpose of the GNU Free Documentation License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or non-commercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

This license is called "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

The Free Software Foundation has designed this license for manuals used in free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this license is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. This license is recommended principally for works whose purpose is instruction or reference.

Online usage
Until June 15, 2009, Wikipedia, and Wikia, Inc. (now Fandom, Inc.) used the 1.2 revision of the GNU Free Documentation License, before switching to the Creative Commons Attribution-ShareAlike 3.0 Unported (CC-BY-SA) license. This unification of licenses applied across the Wikimedia Foundation.

Specific usage
WinUAE provides its documentation under the GNU Free Documentation License.