Open source software is a kind of software which is available for free use and modification, as well as redistribution of both original and modified versions.
The Open Source movement involves people supporting the use of open source licenses as a way to develop better quality software.
Open source vs. Free softwareEdit
While the definition of open source software match very closely with that of free software, their corresponding movements tend to differ with each other at the ideological level. While most open source supporters will be fine with using both open source and proprietary (closed source) software on their machines, followers of the free software movement would not.
Some open source supporters claim "open source" is easier to pitch to businesses in comparison to "free software" (especially since the word "free" may wrongly imply it's impossible to make money off of free software). Given that many companies now release their software as open source without mentioning "free software", they may be right.