The GNU Free Documentation License (GFDL) is a license by the Free Software Foundation desigend for documentation and text.
It is a copyleft license, derivations to text licensed under the GFDL must be likewise licensed under the GFDL.
Like the GPL, the GFDL is incompatible with many other copyleft licenses, include the creative commons share-alike license.
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The LGPL or GNU Lesser General Public License is a Free Software Foundation license that allows for free distribution, with the restriction that modifications to the software must be distributed under the terms of the LGPL as well.
The LGPL however allows linking from non copyleft code and thus is less restrictive than the more popular GNU General Public License.
The LGPL was once termed the Library license instead of the Lesser license, to indicate that the LGPL may be used in circumstances where the viral nature of the license should be restricted to a self-contained library and not the surrounding code.