Free and Open Source Software (also refered to by its acronym 'FOSS', or merely 'Free Software' or 'Open Source Software') are computer programs where the source code — the instructions that tell the computer what to do — are freely shared by programmers, and can be changed and redistributed by other programmers.
Free and Open Source Software are developed both by corporations such as Red Hat and Canonical and by individual hobbyist programmers. Free Software spans the range from mission critical server software through to games and user applications. Notable Free and Open Source Software includes the Linux operating system, the Apache webserver, the Firefox web browser and the OpenOffice.org office suite.
See the Free Software definition for a full description of the properties of Free Software.