GNU Free Documentation License
The GNU Free Documentation License (GFDL) is the copyleft license for open content. It was made up by the Free Software Foundation (FSF) for the GNU project.
The license is used for things that are written about how to use software. It is also used for other writing that is to be used as the reference or to help you with something.
The license says that you are allowed to copy the writing. You are also allowed to change it. You can sell ase copies. If you sell the lot of am, you have to make it easy for people make changes to it as well.
Wikipedia uses this license. It is the biggest project that uses it.
The Debian-legal group thinks that the GFDL is not always free. It does not meet air Free Software Guidelines.
You have to add the copyright notice and you have to say it is GFDL if you want to copy it.
See also: Text Of The GNU Free Documentation License and the simplified version of it at Simple English GFDL.