From Social Patterns
Copyleft is a play on the word copyright to describe the practice of using copyright law to remove restrictions on distributing copies and modified versions of a work for others and requiring that the same freedoms be preserved in modified versions.
Copyleft is a form of licensing and can be used to modify copyrights for works such as computer software, documents, music and art. In general, copyright law allows an author to prohibit others from reproducing, adapting, or distributing copies of the author's work. In contrast, an author may, through a copyleft licensing scheme, give 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. A widely used and originating copyleft license is the GNU General Public License. Similar licenses are available through Creative Commons (Share-alike).
Copyleft licenses are sometimes referred to as viral copyright licenses, because any works derived from a copyleft work must themselves be copyleft when distributed.
Copyleft is considered "strong" when its provisions can be imposed on derived works, and weak when not all derived works inherit the copyleft license.