a copyleft license uses copyright law in order to ensure that every person who receives a copy or derived version of a work can use, modify, and also redistribute both the work, and derived versions of the work
Copyleft is also a movement promoting the use of such licences and critiquing traditional copyright.
For background see Peer Property.
From the Free Software Foundation:
"“To copyleft a program, we first state that it is copyrighted; then we add distribution terms, which are a legal instrument that gives everyone the rights to use, modify, and redistribute the program’s code or any program derived from it but only if the distribution terms are unchanged. Thus, the code and the freedoms become legally inseparable” (http://commoning.wordpress.com/2011/01/04/misunderstanding-the-gnu-general-public-license-reciprocity-in-perpetuity/) ; (FSF 2001).
"A play on the word copyright, copyleft is 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." (http://www.trademarkedsentences.com/en/faq#whatis_copyleft)
“Copyleft uses copyright law, but flips it over to serve the opposite of its usual purpose: instead of a means of privatizing software, it becomes a means of keeping software free. The central idea of copyleft is that we give everyone permission to run the program, copy the program, modify the program, and distribute modified versions— but not permission to add restrictions of their own. For an effective copyleft, modified versions must also be free.” – from The GNU Project, Richard Stallman, (http://www.gnu.org/gnu/thegnuproject.html)
Copyleft describes a group of licenses applied to works such as software, documents, music, and art. Whereas copyright law is seen by the original proponents of copyleft as a way to restrict the right to make and redistribute copies of a particular work, a copyleft license uses copyright law in order to ensure that every person who receives a copy or derived version of a work can use, modify, and also redistribute both the work, and derived versions of the work. Thus, in a non-legal sense, copyleft is the opposite of copyright.
Authors and developers use copyleft with their work to include others in improving and elaborating the work as a continuing process.
"Copyleft is a novel license provision which, thanks to a creative and wise use of copyright law, seems able to permanently affect the development path of digital knowledge assemblages released under its terms. Indeed, if a work is copylefted, everyone can copy, use and modify it, and then distribute the modified versions without asking the copyright holder for permission, as long as the derivative works are also released under the same license terms. These conditions, sometimes derogatively referred to as "viral" in nature, assures that a work is freely available, remains as such, and ensures the same conditions for any improvements or enhancements of the original. These licenses represent a new paradigm in the design and interpretation of intellectual property rights. In the market of digital information goods, this paradigm is competing with the traditional one, copyright. Both aim to solve a certain set of legal and socio–economic issues, namely the appropriation of economic benefits and the promise of a certain life expectancy to collectively created digital goods.
Copyleft does not preclude commercial exploitation of a piece of work. Complementary services and improvements or modifications of the work itself can be sold but the copylefted content will not be subtracted from the conservancy in which it was placed and raised." (http://www.firstmonday.org/issues/issue9_9/ciffolilli/index.html)
Copyleft restriction: "If a creative work carrying this restriction is altered, transformed, or built upon in any way, resulting work may only be distributed under an identical license that includes this restriction. Copyleft is a generational protection of Attribution-ShareAlike. Copyleft means that anyone who changes a work must give the changes back under the same license. It also means that the changes are made known." (http://oerwiki.iiep-unesco.org/index.php?title=OER_glossary)
Christian Siefkes on the three freedoms characteristic of copyleft licenses:
* The freedom to use the work for any purpose. * The freedom to study the work and to change it to make it to do what you wish. * The freedom to distribute and share the work with others, so that the whole community may benefit.
Combinations of the three freedoms must be possible, too. The Free Software Definition specifies a fourth freedom that combines the preceding two (distribute modified versions).
Copyleft ensures that the three freedoms will also hold for all derived works: I may only published derivative versions if I give all their users the same rights." (http://www.keimform.de/2010/self-organized-plenty/)
Copyleft is placed in the history of the opposition to copyright, and seen as distinct from a pure anticopyright stance.
From an essay on the history of author's rights at http://multitudes.samizdat.net/Copyright-Copyleft-and-the.html
Full title: Copyright, Copyleft and the Creative Anti-Commons. A Genealogy of Authors’ Property Rights. Anna Nimus
"Free software guru Richard Stallman claims that in the age of the digital copy the role of copyright has been completely reversed. While it began as a legal measure to allow authors to restrict publishers for the sake of the general public, copyright has become a publishers’ weapon to maintain their monopoly by imposing restrictions on a general public that now has the means to produce their own copies. The aim of copyleft more generally, and of specific licenses like the GPL, is to reverse this reversal. Copyleft uses copyright law, but flips it over to serve the opposite of its usual purpose. Instead of fostering privatization, it becomes a guarantee that everyone has the freedom to use, copy, distribute and modify software or any other work. Its only "restriction" is precisely the one that guarantees freedom — users are not permitted to restrict anyone else’s freedom since all copies and derivations must be redistributed under the same license. Copyleft claims ownership legally only to relinquish it practically by allowing everyone to use the work as they choose as long the copyleft is passed down. The merely formal claim of ownership means that no one else may put a copyright over a copylefted work and try to limit its use.
Seen in its historical context, copyleft lies somewhere between copyright and anticopyright. The gesture by writers of anticopyrighting their works was made in a spirit of generosity, affirming that knowledge can flourish only when it has no owners. As a declaration of "no rights reserved" anticopyright was a perfect slogan launched in an imperfect world. The assumption was that others would be using the information in the same spirit of generosity. But corporations learned to exploit the lack of copyright and redistribute works for a profit. Stallman came up with the idea of copyleft in 1984 after a company that made improvements to software he had placed in the public domain (the technical equivalent of anticopyright, but without the overt gesture of critique) privatized the source code and refused to share the new version. So in a sense, copyleft represents a coming of age, a painful lesson that relinquishing all rights can lead to abuse by profiteers. Copyleft attempts to create a commons based on reciprocal rights and responsibilities — those who want to share the common resources have certain ethical obligations to respect the rights of other users. Everyone can add to the commons, but no one may subtract from it.
But in another sense copyleft represents a step back from anticopyright and is plagued by a number of contradictions. Stallman’s position is in agreement with a widespread consensus that copyright has been perverted into a tool that benefits corporations rather than the authors for whom it was originally intended. But no such golden age of copyright exists. Copyright has always been a legal tool that coupled texts to the names of authors in order to transform ideas into commodities and turn a profit for the owners of capital. Stallman’s idealized view of the origins of copyright does not recognize the exploitation of authors by the early copyright system. This specific myopia about copyright is part of a more general non-engagement with economic questions. The "left" in copyleft resembles a vague sort of libertarianism whose main enemies are closed, nontransparent systems and totalitarian restrictions on access to information rather than economic privilege or the exploitation of labour. Copyleft emerged out of a hacker ethic that comes closest to the pursuit of knowledge for knowledge’s sake. Its main objective is defending freedom of information against restrictions imposed by "the system," which explains why there’s such a wide range of political opinions among hackers. It also explains why the commonality that links hackers together - the "left" in Stallman’s vision of copyleft - is not the left as it’s understood by most political activists.
The GPL and copyleft is frequently invoked as an example of the free software movement’s anticommercial bias. But there is no such bias. The four freedoms required by the GPL — the freedom to run, study, distribute and improve the source code so long as the same freedom is passed down — means that any additional restriction, like a non-commercial clause, would be non-free. Keeping software "free" does not prevent developers from selling copies they’ve modified with their own labour and it also does not prevent redistribution (without modification) for a fee by a commercial company, as long as the same license is passed down and the source code remains transparent. This version of freedom does not abolish exchange — as some free software enthusiasts have claimed — nor is it even incompatible with a capitalist economy based on the theft of surplus value. The contradiction inherent in this commons is partly due to the understanding of proprietary as synonymous with closed-sourced or nontransparent. Proprietary means having an owner who prohibits access to information, who keeps the source code secret ; it does not necessarily mean having an owner who extracts a profit, although keeping the source code secret and extracting a profit often coincide in practice. As long as the four conditions are met, commercial redistribution of free software is nonproprietary. The problem is more obvious when translating this condition to content-based works, like poems, novels, films, or music. If someone releases a novel under a copyleft license, and Random House prints it and makes a profit off the author’s work, there’s nothing wrong with this as long as the copyleft is passed down. To be free means to be open to commercial appropriation, since freedom is defined as the nonrestrictive circulation of information rather than as freedom from exploitation.
It comes as no surprise that the major revision in applying copyleft to the production of artworks, music and texts has been to permit copying, modifying and redistributing as long as it’s non-commercial. Wu Ming claim it is necessary to place a restriction on commercial use or use for profit in order to prohibit the parasitic exploitation of cultural workers. They justify this restriction, and its divergence from the GPL and GFDL versions of copyleft, on the grounds that the struggle against exploitation and the fight for a fair remuneration of labour is the cornerstone of the history of the left. Other content providers and book publishers (Verso, for example) have expanded this restriction by claiming that copying, modifying and redistributing should not only be non-profit but also in the spirit of the original - without explaining what this "spirit" means. Indymedia Romania revised its copyleft definition to make the meaning of "in the spirit of the original" clearer after repeated problems with the neofascist site Altermedia Romania, whose "pranks" ranged from hijacking the indymedia.ro domain to copying texts from Indymedia and lying about names and sources. Indymedia Romania’s restrictions include : not modifying the original name or source since it goes against the desire for transparency, not reproducing the material for profit since it abuses the spirit of generosity, and not reproducing the material in a context that violates the rights of individuals or groups by discriminating against them on the basis of nationality, ethnicity, gender or sexuality since it contradicts its commitment to equality.
While some have multiplied restrictions, others have rejected any restriction at all, including the single restriction imposed by the initial copyleft. It is the movement around peer-to-peer filesharing that comes closest to the gesture of anticopyright. The best example is the Copyriot blog by Rasmus Fleischer of Pyratbiran (Bureau of Piracy), an anti-IP think tank and the one-time founders of Pirate Bay, the most used Bittorent tracker in the P2P community. The motto of copyriot is no copyright, no license. But there is a difference from the older anticopyright tradition. Fleischer claims that copyright has become absurd in the age of digital technology because it has to resort to all sorts of fictions, like distinctions between uploading and downloading or between producer and consumer, which don’t actually exist in horizontal P2P communication. Pyratbiran rejects copyright in its entirety — not because it was flawed in its inception, but because it was invented to regulate an expensive, one-way machine like the printing press, and it no longer corresponds to the practices that have been made possible by current technologies of reproduction.
Stallman’s original definition of copyleft attempts to found an information commons solely around the principle of information freedom — in this sense it is purely formal, like a categorical imperative that demands freedom of information to be universalizable. The only limit to belonging to this community is those who do not share the desire for free information — they are not excluded, they refuse to participate because they refuse to make information free. Other versions of copyleft have tried to add further restrictions based on a stronger interpretation of the "left" in copyleft as needing to be based not on a negative freedom from restrictions but on positive principles like valuing social cooperation above profit, nonhierarchical participation and nondiscrimination. The more restrictive definitions of copyleft attempt to found an information commons that is not just about the free flow of information but sees itself as part of a larger social movement that bases its commonality on shared leftist principles. In its various mutations, copyleft represents a pragmatic, rational approach that recognizes the limits of freedom as implying reciprocal rights and responsibilities — the different restrictions represent divergent interpretations about what these rights and responsibilities should be. By contrast, anticopyright is a gesture of radicality that refuses pragmatic compromises and seeks to abolish intellectual property in its entirety. Anticopyright affirms a freedom that is absolute and recognizes no limits to its desire. The incompatibility between these positions poses a dilemma : do you affirm absolute freedom, knowing it could be used against you, or moderate freedom by restricting the information commons to communities who won’t abuse it because they share the same "spirit" ?" (http://multitudes.samizdat.net/Copyright-Copyleft-and-the.html)
More info at http://www.gnu.org/copyleft/copyleft.html
A primer on the ethics of intellectual property law, at http://www.ram.org/ramblings/philosophy/fmp/copying_primer.html