I am not sure about all of them. But if they are pink/light red, you have not yet read them.
If they have a blue arrow pointing up to the left, they have been moved from another forum.
If they have a little padlock on them, then that topic has been locked by a Moderator, and no further discussion can take place in that thread.
The ones that look like a little light bulb are sticky posts -- they always stay up at the top of the forum.
And the round ones with "i" in them are announcements. They also sit up top, above the sticky posts.
I don't know what the little red star on only some of them indicates.