Open Source qualifies a software of which the source code is available to the general public for use and/or modification.
Open source code is typically created as a collaborative effort in which programmers improve upon the code and share the changes within the community. Open source sprouted in the technological community as a response to proprietary software owned by corporations.
Two main types of open source software may be free and non-free (shared source). Open Source is not the same as free software, see http://www.gnu.org/philosophy/free-software-for-freedom.html