Free Software and Open Source
The terms Free Software and Open Source usually refer the same software (like the Linux kernel, the Apache webserver, or the MySQL database), but they also carry political and philosophical meanings within the community.
Free Software
The Free Software movement was founded in 1984 by Richard Stallman when he started the GNU project. The word Free was about freedom, not price; or free as in "free speech", not as in "free beer".
Stallman thought that freedom was important. For software he listed four freedoms:
- (0) to run the program, for any purpose.
- (1) to study how the program works, and adapt it to your needs.
- (2) to redistribute copies so you can help your neighbor.
- (3) to improve the program, and release your improvements to the public, so that the whole community benefits.
Access to the source code is required for freedoms #1 and #3.
When somebody uses the term Free Software they probably want to focus on the freedom of the user, or they started working with the software before 1998.
Open Source
In 1998 (14 years after Free Software) some Free Software developers, including Bruce Perens, invented the term Open Source and formed the Open Source movement because they wanted to focus on the practical benefits of Free Software. Some people prefer Open Source because they dislike the ambiguity of the word Free.
When somebody uses the term Open Source they probably want to focus on the practical benefits of the software.
FOSS
Many people developing and using Free Software / Open Source want to hold a neutral position so they prefer to use acronyms.
FOSS is an acronym for Free Open Source Software. To be accurate we really need more punctuation: maybe [Free / Open Source] Software; or something that makes it clear that the Software is both Free Software and Open Source Software.
Because both phrases are often misunderstood in English some people prefer to call it Software Libre and so use the acronum FLOSS.