Open Source Software can be defined as software for which the human-readable source code is made available under a copyright license (or arrangement such as the public domain) that meets the Open Source Definition. This permits users to use, change, and improve the software, and to redistribute it in modified or unmodified form. It is very often developed in a public, collaborative manner. (Definition taken from Wikipedia: http://en.wikipedia.org/wiki/Open-source_software).

You can learn more about open source at LibLime's Website.