A matter of copyright
In compliance with international law, software is covered by copyright. This is the basis on which licenses are able to guarantee Free Software’s four freedoms to everybody. There are several Free Software licenses, depending on the context in which they’re used, the goals, and the type of software. The identity of a free program is thus defined by its Free Software license. The latter specifies the rights and duties of each party. Legally, they are based on copyright and aim to reverse its principle: authorise rather than restrict.
Among the Free Software licenses, some are said to be copyleft. The principle of copyleft is to give everybody the possibility to use a free work while ensuring that the freedoms that its license provides are preserved. Beyond the four freedoms of Free Software, the copyleft licenses require keeping the same license in case the work is redistributed, whether or not it has been modified. This type of license enables real sharing of creation and knowledge, and ensures that cultural works will remain free. The aim of copyleft licenses is to enrich the common trove of works that are available to all—a trove to which everyone can add, but from which no one may subtract.