The GNU General Public License (GNU GPL or simply GPL) is an FSF approved free software license and currently the most popular license for free software (or "open source software").

The GPL grants the following rights, or "freedoms":

• The freedom to run the program, for any purpose.
• The freedom to study how the program works, and modify it. (Access to the source code is a precondition for this.)
• The freedom to redistribute copies.
• The freedom to improve the program, and release the improvements to the public. (Access to the source code is a precondition for this.)

Full text: GNU GPL (Current version: v2, 1991)

The GNU Lesser General Public License (formerly the GNU Library General Public License) is an FSF approved free software license designed as a compromise between the GNU GPL (see above) and simple permissive licenses such as the BSD license and the MIT License.

The main difference between the GPL and the LGPL is that the latter can be linked to a non-(L)GPLed program, which may be free software or proprietary.

Full text: GNU LGPL (Current version: v2.1, 1999)

The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for free content, designed by the FSF for the GNU project. It is the open content counterpart to the GNU GPL (see above).

The license was designed for manuals, textbooks, and other reference and instructional materials and can be used for any text-based work, regardless of subject matter. It stipulates that any copy of the material, even if modified, carry the same license. Those copies may be sold but, if produced in quantity, have to be made available in a format which facilitates further editing.

Full text: GNU FDL (Current version: v1.2, 2002)