In a nutshell, Open Source means that the source code for the software is open. The theory is that by Opening up the source code you get a huge bevy of bug-testers and security-checkers, so the resulting product will be stronger than any closed, proprietary system

Built upon that attribute there are many different licenses. A popular one is The GNU General Public License (GPL), which allows free use, distribution and alteration, so long as the results of these diistributions or alterations are released under the same license.

Open Source Software is not necessarilyFree Software, as Open Source refers to the process and noit necessarily the rules of distribution. In fact, there are many Categories of Free and Non-Free Software.