Copyleft

From: http://en.wikipedia.org/wiki/Copyleft

Copyleft describes a group of licenses applied to works such as software, documents, music, and art. Whereas 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.

Authors and developers use copyleft with their work to include others in improving and elaborating the work as a continuing process.

More info at http://www.gnu.org/copyleft/copyleft.html