Thread Icons

Is there an explanation of the various thread icons (envelope icons) on the site? I've nosed around a bit, but haven't found anything. I think it's about time I learn what they mean.

http://www.linuxquestions.org/questi...p?s=&forumid=7

Scroll to bottom.. you'll see descriptions. Basically on main page of each forum.

Thanks. It's been staring me in the face since I started here, but I never saw it