Computer software provided by a creator or community for consumers to freely inspect, modify, enhance, and distribute the software and its source code. It is released under a licensing model known as Copyleft, where the copyright holder confers obligations to a consumer using that work, and all subsequent derivative works.
When a license requires that only a portion of the derivitive work that is OSS be distributed under the copyright holder's license, it is considered Weak Copyleft. Examples of Weak Copyleft licenses include Apache, BSD, MIT, and MS-PL. Typically, use of Weak Copyleft software requires only that the consumer of the software include the license and attribute the copyright holder when distributing the derivative work.
When a license requires that the entire derivative work be distributed under the copyright holder's license it is considered Strong Copyleft. Examples of Strong Copyleft licenses include GPL, LGPL, Mozilla, and MS-RL. The viral nature of Strong Copyleft software should be considered when applying it to a work that contains intellectual property or proprietary code, since there is often a requirement for publication of the entire source.
Open Source Software includes products that are also considered Free Software, however, just because a software is OSS, does not mean it is free of cost for licensing or support.