What does “GPL” stand for?
“GPL” stands for “General Public License”. The most widespread such license is the GNU General Public License, or GNU GPL for short. This can be further shortened to “GPL”, when it is understood that the GNU GPL is the one intended.

Does free software mean using the GPL?
Not at all—there are many other free software licenses. We have an incomplete list. Any license that provides the user certain specific freedoms is a free software license.

Why should I use the GNU GPL rather than other free software licenses?
Using the GNU GPL will require that all the released improved versions be free software. This means you can avoid the risk of having to compete with a proprietary modified version of your own work. However, in some special situations it can be better to use a more permissive license.

Does all GNU software use the GNU GPL as its license?
Most GNU software packages use the GNU GPL, but there are a few GNU programs (and parts of programs) that use looser licenses, such as the Lesser GPL. When we do this, it is a matter of strategy.

Does using the GPL for a program make it GNU software?
Anyone can release a program under the GNU GPL but that does not make it a GNU package.

Making the program a GNU software package means explicitly contributing to the GNU Project. This happens when the program's developers and the GNU Project agree to do it. If you are interested in contributing a program to the GNU Project, please write to <maintainers@gnu.org>.

What should I do if I discover a possible violation of the GPL?
You should report it. First, check the facts as best you can. Then tell the publisher or copyright holder of the specific GPL-covered program. If that is the Free Software Foundation, write to <license-violation@gnu.org>. Otherwise, the program's maintainer may be the copyright holder, or else could tell you how to contact the copyright holder, so report it to the maintainer.

Why does the GPL permit users to publish their modified versions?
A crucial aspect of free software is that users are free to cooperate. It is absolutely essential to permit users who wish to help each other to share their bug fixes and improvements with other users.

Some have proposed alternatives to the GPL that require modified versions to go through the original author. As long as the original author keeps up with the need for maintenance, this may work well in practice, but if the author stops (more or less) to do something else or does not attend to all the users' needs, this scheme falls down. Aside from the practical problems, this scheme does not allow users to help each other.

Sometimes control over modified versions is proposed as a means of preventing confusion between various versions made by users. In our experience, this confusion is not a major problem. Many versions of Emacs have been made outside the GNU Project, but users can tell them apart. The GPL requires the maker of a version to place his or her name on it, to distinguish it from other versions and to protect the reputations of other maintainers.