As described in the Open Source Initiative's (http://www.opensource.org/docs/definition.php Definition), Open Source software is software that has:

** Free Redistribution
** Source Code
** Licensing in regards to:
*** derived works
*** integrity of the source code
...and so on

However, this isn't unanimous amoungst the open source community. The (http://www.gnu.org Free Software Foundation) has a (http://www.gnu.org/philosophy/philosophy.html#AboutFreeSoftware slightly different view). You can check out their licenses (http://www.gnu.org/licenses/licenses.html at this address). Regarded as pioneers of the open source movement, this organisation is behind the popular GPL licence that most linux distributions are released under.