Copyleft is the legal technique of granting certain freedoms over copies of copyrighted works with the requirement that the same rights be preserved in derivative works. In this sense, freedoms refers to the use of the work for any purpose, and the ability to modify, copy, share, and redistribute the work, with or without a fee. Licenses which implement copyleft can be used to maintain copyright conditions for works ranging from computer software, to documents, art, and scientific discoveries, and similar approaches have even been applied to certain patents.
Copyleft software licenses are considered protective or reciprocal (in contrast with permissive free software licenses): they require that information necessary for reproducing and modifying the work be made available to recipients of the software program. 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. Copyleft helps ensure everyone's rights to freely use the product but it prohibits owning, registering copyright and earning royalties from copyright.