As mentioned, the forums and threads you haven't read will be bolded; those you have read
completely will appear unbold. If a single thread in a forum or post in a thread has not been read, the entire thread/forum will remain bolded for you.
There is also a little dot to the left of the thread title (between the avatar and the title) that indicates new unread posts. That dot appears to be sort of two-toned blue on my screen -- someone told me once that it's actually a face with glasses, but I can't tell that unless I zoom in real big.
A red pushpin to the far right of a thread title, just before the number of replies, indicates that thread is "pinned" or "stuck" to the top of that forum. It will remain near the top, even if there are no new posts. These are usually informational threads, stuff that kind of stands the test of time and doesn't change, and doesn't usually generate much discussion.
A gold padlock symbol indicates the thread has been locked and no further replies may be posted.
The red box with a white number next to the words "Inbox" or "Alerts" near the top left indicates how many new/unread PMs (conversations) or Alerts you have received since you last clicked or hovered over that area.
The larger avatar to the far left of a thread belongs to the person who started that thread. If you have posted on that thread, you'll see your own avatar as a smaller box along with the thread-starter avatar. If you started the thread and have subsequently posted to it, you'll see a "double" avatar (both large and small the same).
A green mark in the top left corner of an avatar indicates that poster is currently online.
A blue arrow to the right of a thread title is a re-direct and means that thread has been moved to a different forum. If you click on the thread it will automatically take you to the thread in the new location.
Near the top and bottom of your screen is a gray bar (might be a different color if you have a different style applied) with "Home" "Forums" and the forum you are currently in. At the far right of that bar is a bluish arrow pointing towards the top right. That is a quick navigation link -- if you click it a pop-up will appear with all the forums as well as some other links (account profile, conversations, etc.). It's a way to quickly move from one part of the boards to another without multiple clicks back through the main home page.
That's all I can think of at the moment. If you find another symbol or icon and don't know what it means, feel free to ask!
