The GFDL, notable for once being the lifeblood of Wikipedia, is designed by the Free Software Foundation; it is the copyleft license of choice for many wikis and manuals. However, Debian doesn't deem it free enough.
What does the GFDL mean?
GFDL stands for the GNU Free Documentation License. This license allows one to edit, redistribute and generally do whatever the fuck you want to do with anything that is licensed under the GFDL, as well as any derivative works of the original document. This basically gives you unlimited freedom. Jews hate it.
Drama and lulz
Considerable drama can be generated on websites, such as the YTMND Wiki, if one makes a sizable number of edits and then revokes the license on them, thus forbidding the website from distributing your work. Malber stuck it to Wikipedia (when the GFDL was still in use) thusly:
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