Tardis:Copyleft

Copyleft describes a group of licenses applied to works such as software, documents, and art. Where copyright law is seen by the original proponents of copyleft as a way to restrict the right to make and redistribute copies of a particular work, a copyleft license uses copyright law in order to ensure that every person who receives a copy or derived version of a work, can use, modify, and also redistribute both the work, and derived versions of the work. Thus, in a non-legal sense, copyleft is the opposite of copyright.

Wikipedia has an entry for "copyleft" which gives a more detailed background on this concept.