A play on the word "copyright" this is the practice of using copyright law to remove restrictions on distributing copies and versions of a work and requiring that the same freedoms be preserved in modified versions. In the online world, it is a form of licensing and may be used to modify copyrights for works such as computer software, documents, music, and art. The main effect is that through a "copyleft" licensing scheme, an author gives every person who receives a copy of a work permission to reproduce, adapt, or distribute the work as long as any resulting copies or adaptations are also bound by the same copyleft licensing scheme. Similar licenses are available through Creative Commons (called Share-alike); see the GNU definition for GPL, a widely used and originating copyleft license.
The copyleft symbol above is the "reversed c in a full circle" and is the copyright symbol turned in the left direction. (Unlike the copyright symbol it has no legal meaning.)