Copyleft is the practice of using copyright law to offer the right to distribute copies and modified versions of a work and requiring that the same rights be preserved in modified versions of the work. In other words, copyleft is a general method for making a creative work freely available to be modified, and requiring all modified and extended versions of the creative work to be free as well.
Copyleft is a form of licensing and can be used to maintain copyright conditions for works such as computer software, documents, and art. In general, copyright law is used by an author to prohibit recipients from reproducing, adapting, or distributing copies of the work. In contrast, under copyleft, an author may give every person who receives a copy of a work permission to reproduce, adapt or distribute it and require that any resulting copies or adaptations are also bound by the same licensing agreement.
Copyleft licenses (for software) require that information necessary for reproducing and modifying the work must be made available to recipients of the executable. The source code files will usually contain a copy of the license terms and acknowledge the author(s).
Copyleft type licenses are a novel use of existing copyright law to ensure a work remains freely available. The GNU General Public License, originally written by Richard Stallman, was the first copyleft license to see extensive use, and continues to dominate the licensing of copylefted software. Creative Commons, a non-profit organization founded by Lawrence Lessig, provides a similar license provision condition called ShareAlike.
More on Wikipedia: http://en.wikipedia.org/wiki/Copyleft