Copyleft is the practice of granting the right to freely distribute and modify intellectual property with the requirement that the same rights be preserved in derivative works created from that property. Copyleft in the form of licenses can be used to maintain copyright conditions for works ranging from computer software, to documents, art, scientific discoveries and even certain patents.
Copyleft software licenses are considered protective or reciprocal in contrast with permissive free software licenses, and require that information necessary for reproducing and modifying the work must be made available to recipients of the software program, or binaries. This information is most commonly in the form of source code files, which usually contain a copy of the license terms and acknowledge the authors of the code.
Notable copyleft licenses include the GNU General Public License (GPL), originally written by Richard Stallman, which was the first software copyleft license to see extensive use, the Mozilla Public License, the Free Art License and the Creative Commons share-alike license condition, the last two of which being intended for other forms of intellectual and artistic work, such as documents and pictures.