Okay, I will give it out brief. Free in the Free Software as with its philosophy defines "free" as in "free thoughts" and not as in "free beer". Essentially what makes a software Open Source or Closed Source or Free Software is determined by the license that it carries. But when most of the licenses (almost all the popular ones) that qualify for "Open Source" also qualify for the "Free Software", essentially the software that carries such a license is essentially Open Source as well as Free Software.
NB: If you don't understand something as you read on, just open up a new tab on your browser and do a search on the subject, you will find so many good links. So much has been already written on the subject that I find it pointless to write them again. Please guess it right that I am too lazy to give out those links here. Hope you can excuse me for that!
Even while most of the software qualify under both categories, its authors and promoters try to push it as Open Source, and not Free Software. Why? Popularity ... branding your software as being "Open Source" makes it more popular than branding it as "Free Software". Try to imagine a software that you have used and was promoted as "Free Software". Can't gain much... right?
I am not being paid by Eric S Raymond or OSI, neither by Richard Stallman or GNU or FSF, so I should make a clear stand on both. While there are very narrow differences in the qualified Licenses for the two, they differ in their philosophies to where the emphasis goes. While the Open Source philosophy emphasis is on the openness of the source (of the software), Free Software emphasis is on the liberty and freedom on the software that you use. As the FSF/GNU site mentions it, Free Software and Open Source are not enemies, but rather closed source is the enemy.
There are good number of essays on this topic, but I thought of writing this post to save some of your time reading it. But if you are a bit more free, just read this one to understand some differences between the two philosophy. (Okay, this will not explain why most people prefer an Open Source branding as such, though it explains that "open source" is considered as a "marketing" campaign for free software).
There is so much to write on the subject, but this is a comparatively small post because:
I try to promote FLOSS as I deal with people personally, but for others, some of my posts like this should help. Just post on a comment if it helped a penny on your thoughts to know FOSS.