I am working on fixing up an old mountain bike that I found in the trash. When I took it apart, I couldn't figure out how to get the crank off, so I took it to my LBS and asked them to do it for me. I figured once it was out it would be simple to understand how it worked, but they seem to have put it back together for me and I'm stumped. I googled crank removal, but my crank doesn't look like any of the ones I see in the tutorials.

Can someone tell me what this kind of crank is called, and how you take it apart?