Copyleft is a general method for making a program (or other work) free, and requiring all modified and extended versions of the program to be free as well.
The simplest way to make a program free software is to put it in the public domain, uncopyrighted. This allows people to share the program and their improvements, if they are so minded. But it also allows uncooperative people to convert the program into proprietary software. They can make changes, many or few, and distribute the result as a proprietary product. People who receive the program in that modified form do not have the freedom that the original author gave them; the middleman has stripped it away.
The Copyleft concept was originated by the GNU Project, sponsored by the Free Software Foundation.
The term Copyleft is a play on the term copyright, which the GNU Project claims has the purpose of restricting a users' rights to the software that he acquires. In contrast, Copyleft is designed to maintain and protect a user's rights. Two freedoms are usually associated with Copyleft: the freedom to acquire the software without paying for it, and the freedom to modify and redistribute the software. These two freedoms are often referred to as "free as in beer", and "free as in speech". Most copylefted works are released under the traditional GPL licence. However, debate has been building in recent years as to the validity of the license, and there are now several licences to choose from. Most of these licences have been officially translated into many different languages, although the original English translation is intended to be the licence used in court if the need arises.
Copyleft Definition : is a play on the word copyright to describe 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.
Common copylefted software include Mozilla's Firefox web browser, the Open Office suite of productivity tools, and the GNU/Linux operating system. In addition, the entire Wikipedia website is copylefted, including the source code of the operating system that runs the server, the webserver software itself, the programs that create the webpages, and the content of the webpages.
Copyright Definition: The exclusive legal right, given to an originator or an assignee to print, publish, perform, film, or record literary, artistic, or musical material, and to authorize others to do the same
A document granting exclusive right to publish and sell literary or musical or artistic work
Copyright protects the physical expression of ideas. As soon as an idea is given physical form, e.g. a piece of writing, a photograph, music, a film, a web page, it is protected by copyright. There is no need for registration or to claim copyright in some way, protection is automatic at the point of creation. Both published and unpublished works are protected by copyright.
Copyright is normally owned by the creator(s) of the work, e.g. an author, composer, artist, photographer etc. If the work is created in the course of a person's employment, then the copyright holder is usually the employer.
Copyright is a property right and can be sold or transferred to others. Authors of articles in academic journals, for example, frequently transfer the copyright in those articles to the journal's publisher. It is important not to confuse ownership of a work with ownership of the copyright in it: a person may have acquired an original copyright work, e.g. a painting, letter or photograph, but unless the copyright in it has expressly also been transferred, it will remain with the creator.
Copyright, Designs and Patents Act 1988
Copyright owners have exclusive rights to their work which include the right:
To copy work
To issue copies of the work to the public
To perform, show or play the work in public
To broadcast the work or include it in a cable programme service
To adapt the work or do any of the above in relation to the adaptation.