Great amount of licenses exists of free software and its agreement can be very diverse. Despite of general way we can divide the licenses in two great blocks according to its more important characteristics. First they would be those that do not impose any condition in his second redistribution (permissive licenses) and second they would be those that do yes it (robust licenses or copyleft). Copyleft initially was developed for the distribution of the computer science programs. Software, nowadays so known, as Linux, Open Office or FireFox were created like free programs under copyleft.
With time, copyleft has extended to very diverse scopes, besides the computer science one, also to music, the edition, the right, the art or the media. One of first and more known projects than restored the bases of the conditions for the licenses of copyleft was the project of free software devised by company GNU. It was in 1983 when Richard Stallman created a productive process of cooperation for the elaboration of a free operative program of any restriction derived from copyright. The project mainly consisted of the elaboration of a set of licenses that guaranteed that the software created by the equipment of Stallman stayed completely frees in all later versions. The participants in this process could use, share, modify or improve anyone of the free versions of this software being forced to put it at the disposal of the rest of the society without no restriction regarding their operation. Of this form he was wanted to prevent that once put to the service of all the community of users without no restriction somebody he could change these conditions. Unlike a program under the public dominion in which also changes can be realised, copyleft prevents that it turns somebody it into a privative program. In a software under the public dominion any person who introduced modifications in the same could distribute, it as own product, that is to say, the intermediary could break the chain of free use initiated by the author original and to begin to distribute the versions modified under its own copyright. In case of copyleft, whatever it uses the free program it tries and it to redistribute, with or without changes, it will have to grant to the following receiver the freedom to copy it or to modify it, guaranteeing at any moment the maintenance of these initial conditions.