GNU Free Documentation License
The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for free documentation, designed by the Free Software Foundation (FSF) for the GNU project. It is the counterpart to the GNU GPL that gives readers the same rights to copy, redistribute and modify a work and requires all copies and derivatives to be available under the same license. Copies may also be sold commercially, but if produced in larger quantities (greater than 100) then the original document or source code must be made available to the work's recipient.

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.

Secondary sections
The license explicitly separates any kind of "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 (but both-ways incompatible with) the GNU General Public License, 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.

Commercial redistribution
The GFDL requires the ability to "copy and distribute the Document in any medium, either commercially or noncommercially" and therefore is incompatible with material that excludes commercial re-use. Material that restricts commercial re-use is incompatible with the license and cannot be incorporated into the work. However, incorporating such restricted material may be fair use under United States copyright law and does not need to be licensed to fall within the GFDL if such fair use is covered by all potential subsequent uses. One example of such liberal and commercial fair use is parody.

Criticism of the GFDL
The Debian project and Nathanael Nerode have raised objection. Debian developers eventually voted to consider works licensed under the GFDL to comply with their Debian Free Software Guidelines provided the invariant section clauses are not used. These critics recommend the use of alternate licenses such as the share-alike Creative Commons licenses, the BSD Documentation License, or even the GNU GPL. They consider the GFDL a non-free license. The reasons for this are that the GFDL allows "invariant" text which cannot be modified or removed, and that its prohibition against digital rights management (DRM) systems applies to valid usages, like for "private copies made and not distributed".

Overly broad DRM clause
The GNU FDL contains the statement:

A criticism of this language is that it is too broad, because it applies to private copies made but not distributed. This means that a licensee is not allowed to save document copies "made" in a proprietary file format or using encryption.

In 2003, Richard Stallman said about the above sentence on the debian-legal mailing list:

Invariant sections
A GNU FDL work can quickly be encumbered because a new, different, title must be given and a list of previous titles must be kept. This could lead to the situation where there are a whole series of title pages, and dedications, in each and every copy of the book if it has a long lineage. These pages cannot ever be removed, at least not until the work enters the public domain after copyright expires.

Richard Stallman said about invariant sections on the debian-legal mailing list:

GPL incompatible in both directions
The GNU FDL is incompatible in both directions with the GPL: that is GNU FDL material cannot be put into GPL code and GPL code cannot be put into a GNU FDL manual. Because of this, code samples are often dual-licensed so that they may appear in documentation and can be incorporated into a free software program.

At the Juun 22nd and 23rd international GPLv3 conference in Barcelona, Moglen hinted that a future version of the GPL could be made suitable for documentation:

Burdens when printing
The GNU FDL requires that licensees, when printing a document covered by the license, must also include "this License, the copyright notices, and the license notice saying this License applies to the Document". This means that if a licensee prints out a copy of an article whose text is covered under the GNU FDL, he or she must also include a copyright notice and a physical printout of the GNU FDL, which is a significantly large document in itself.

Transparent formats
The definition of a "transparent" format is complicated, and may be difficult to apply. For example, drawings are required to be in a format that allows them to be revised straightforwardly with "some widely available drawing editor." The definition of "widely available" may be difficult to interpret, and may change over time, since, e.g., the open-source Inkscape editor is rapidly maturing, but has not yet reached version 1.0. This section, which was rewritten somewhat between versions 1.1 and 1.2 of the license, uses the terms "widely available" and "proprietary" inconsistently and without defining them. According to a strict interpretation of the license, the references to "generic text editors" could be interpreted as ruling out a format used by an open-source word-processor such as Abiword; according to a loose interpretation, however, Microsoft Word .doc format could qualify as transparent, since a subset of .doc files can be edited perfectly using OpenOffice.org, and the format therefore is not one "that can be read and edited only by proprietary word processors."

History
The FDL was released in draft form for feedback in late 1999. After revisions, version 1.1 was issued in Maachị 2000, and version 1.2 in Novemba 2002. The current state of the license is version 1.2.

The first discussion draft of the GNU Free Documentation License version 2 was released on 26 Septemba 2006, along with a draft of the new GNU Simpler Free Documentation License.

The new draft of the GNU FDL includes a number of improvements, such as new terms crafted during the GPLv3 process to improve internationalization, clarifications to help people applying the license to audio and video, and relaxed requirements for using an excerpt from a work.

The new proposed GNU Simpler Free Documentation License has no requirements to maintain Cover Texts and Invariant Sections. This will provide a simpler licensing option for authors who do not wish to use these features in the GNU FDL.

Notes

External links
 GFDL official text
 The GNU Free Documentation License
 Free Software and Free Manuals
 Draft of Debian position statement about the GFDL
 Why You Shouldn't Use the GNU FDL
 Why Wikivoyage isn't GFDL: Problems with using the GFDL for short printed texts
 The Free Universal Encyclopedia And Learning Resource
 Guide to the new drafts of documentation licenses

 
Wikipedia