I think you may be interested in this article by Richard Stallman, founder of GNU, the free software foundation and author of the GPL, among other great accomplishments:
https://www.gnu.org/philosophy/open-source-misses-the-point.html
I didn’t re-read it right now, but as I understood it the main difference between Free Software and Open Source Software is that you release Free Software because you believe in Freedom and you release Open Source Software because you think it’s a nice tool to get Crowd Involvement and don’t have to spend so much money on professionals.
So it’s a philosophical difference, while technically there is no real difference (Free Software is always Open Source and Open Source Software (usually) grants (almost) all Freedoms necessary to call it Free Software).
I voted for Free Software, because I believe in the philosophy behind it, but usually I use the term FLOSS as a hypernym for both (the L is important - as “free libre” makes it clear that free is used as in free speech and not as in free beer).