What is software licensing?
Software licences tell people how the rights holder (usually the author) wants the software to be used and what freedoms or restrictions it has. Without a licence, many uses of the software may be prohibited.
Free Software licenses are licenses that give permission for a user to obtain four important freedoms; the freedom to use, modify, share and improve software. Copyleft licenses are Free Software licences that allow the four fundamental freedoms under the condition that those freedoms remain intact in further distribution of the software or derivative works.
The GNU GPL is the most famous Free Software copyleft license and is used by a large percentage of Free Software projects. It has many useful features, like ensuring the four freedoms and providing a limited warranty and disclaimer of liability clause to protect developers releasing their work to the public.
There are many Free Software licenses, some without any copyleft effect. Some of these are compatible with the GNU GPL, some have special provisions that enable compatibility, some are incompatible.
If people use the same licence or at least compatible licences then projects can exchange code freely. We encourage everyone to use the GNU GPL for all software.