The open source model allows user to view and modify a product’s source code. Organizations and individuals that uses this model believes that the benefit that they gain from improvements to their software provided by the community of software developers is more important than protecting their competitive advantage. Most advantages cited by proponents for having such a structure are expressed in terms of trust, acceptance, teamwork and quality. Most of the open source software is licensed under what is often termed a “copyleft” license[citation needed], a term which gives emphasis to the license’s reversal of the principles of copyright. A traditional license is used to limit freedoms, which the free software movement considers importantl, the “four software freedoms”, taking them away from the users either completely (“you may not distribute the software”) or partially (“you can use the software for an evaluation period of 30 days; after that you must either pay a license registration fee or discontinue the software”). By contrast a copyleft license protects the “four software freedoms” by granting them and then explicitly prohibiting anyone to strip them away when redistributing the package or reusing the code in it to make derivative works. But there are few open source software packages licensed under a license that grants the four software freedoms but allows re distributors to remove them if they intend to do so. Such licenses are sometimes called permissive software licenses. An example of such a license is the BSD license which allows derivative software to be distributed as closed source products, as long as they give credit to the original designers.
(Source:The Wikipedia)