WordPress : free software or open source (semantics?)

WordPress consulted Free Software lawyers about the GPL license in regards to themes and graphics in wordpress.

The quote from the lawyer was a little long so I didn’t read it. But I did read through the free software manifesto on the GNU website.

On that page they have a link to an article on the difference between open source and free software.

I’ve read quite a bit about the free software and the open source movements in various books and thought I knew something of the difference. I also have been using open source CMS for my websites since 2003 and I have been using and monkeying around with GNU Linux (asterisk project|puppy linux) as my operating system of choice since late 2007 (although I have known and used it as far back as 2002 – RedHat).

What is the difference? I’m not really sure.
I know that free software (software released under GPL) is open source, as in you have full access to the source code.
But I am guessing that software claiming to be open source may not necessarily be GPL free software if it does not give all four of the freedoms offered under GPL.