Open source definition:
http://opensource.org/docs/osd
Free software defintion:
http://www.gnu.org/philosophy/free-sw.html
Both are talking about free-as-in-speech. FSF is "more free" in that for software to meet the FSF standards, it must afford more freedoms to its users. The OSI standards are looser, "free" software is "open" but the reverse isn't necessarily true - It turns out even this isn't always true.
They are functionally the same in the vast majority of cases. It's a philosophical difference - FSF wants intellectual freedom, OSI wants practical freedom to (re)use and adapt software.
