From COKS - Open Source Center - Slovenia
An open source license is a copyright license for computer software that makes the source code available under terms that allow for modification and redistribution without having to pay the original author. Such licenses may have additional restrictions such as a requirement to preserve the name of the authors and the copyright statement within the code. One popular (and sometimes considered normative) set of open source software licenses are those approved by the Open Source Initiative (OSI) based on their Open Source Definition (OSD).
There are also shared source licenses, such as the Microsoft Permissive License (MS-PL) and Microsoft Community License (MS-CL), which have some similarities with open source. Other shared source licenses, such as the Microsoft Reference License (MS-RL), are not compatible with the Open Source Definition.
The Free Software Foundation has related but distinct criteria for evaluating whether or not a license qualifies a program as free software. Likewise, the Debian project has its own criteria, the Debian Free Software Guidelines, on which the Open Source Definition is based.
OSI approved licenses
Software in the public domain (that is, with all copyright renounced), meets those criteria as long as all source code is made available, and is therefore recognized by the OSI and entitled to use their service mark. In addition, OSI has approved the following licenses (link):