*Open Source:* When programmers can read, redistribute, and modify the source code for a piece of software. *Free Software:* Free software is a matter of liberty, not price. Users of free software are free to run, copy, distribute, study, change and improve the software. *Proprietary Software:* In the context of Free and Open Source Software (FOSS), proprietary software is software that is not licensed under a FOSS license. *Commercial Software:* Software that is sold. FOSS software can also be commercial.