Copyleft

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

See for examples, the General Public License, some versions of the Creative Commons licences, and the more radical IANG License.

For background see Peer Property.

Description

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 Information

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

A primer on the ethics of intellectual property law, at http://www.ram.org/ramblings/philosophy/fmp/copying_primer.html