Inspired by the very open geek culture surrounding the MitMediaLab in the 60's (?) the FreeSoftwareFoundation was founded by RichardStallman (aka RMS) in an attempt to create an entirely free computer operating system and the applications to run on it.
The FreeSoftwareFoundation has a highly political agenda with uncompromising and critical attitudes towards non-free software. Their catch phrase is "Free as in freedom not free as in beer", which is intended to clarify their usage of the word "free". The GnuPublicLicense (aka. the GPL) is a legal frame work for releasing FreeSoftware and is the legal embodiment of the freedoms they hold dear.
Confusingly FreeSoftware is often referred to as OpenSource. While they both create a similar end products the OpenSource movement was largely an attempt make FreeSoftware paletable to the commercial world by rebranding it and removing RichardStallman's contentious personality.
To everyones dissatisfaction the terms OpenSource and FreeSoftware are basically synonymous these days.