Correct me if I am wrong, but I will take a stab at this.
This means there are no new threads or post since your last visit to this forum.
There have been new threads since made. (Or post)
THe dot signifies that you have "subscribed to this thread" and it will email you when a new reply is made onit.
The closed letter is explained below this.
This is a thread that you have viewed and has not since had any new replies within it since your last view.
This is a thread that you have allready once opened, but has since had a new reply.
This is a thread that you have opened and has a new replies within it. It is red because it considered "hot" and has a lot of replies.
Was once a booming thread that has been since locked.
Hope that helps somewhat.