Different types of GNU licence

GNU is a recursive acronym for "GNU's Not Unix". The GNU project was announced in 1983 by Richard Stallman with the goal of creating a complete operating system -- called the GNU system or simply GNU -- that is free software, meaning that users are allowed to copy, modify and redistribute it. The GNU project is now carried out under the auspices of the Free Software Foundation (FSF).

There are 3 types of GNU licence.

1. GPL - GNU General Public Licence

Under this licence, disclosure of source code to the target audience is necessary.

2. LGPL - GNU Lesser General Public License

It applies to certain designated libraries, and
is quite different from the ordinary General Public License. We use
this license for certain libraries in order to permit linking those
libraries into non-free programs.

3. GFDL - GNU Free Documentation License

The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially.

Reference: http://www.gnu.org/licenses/