When looking at the topic (thread) list in any of the forums, at the left of each thread is an image (icon) that represents the thread status.
These statuses include:
Read - you have viewed the thread since the last time anyone posted to it, there are no new posts (white background)
Unread - There has been at least one new post to the thread since you last viewed the thread (yellow background)
Hot - The thread has more than some number of replies, the Hot threshold is configurable by an admin only (flame)
Yours - Threads that you have posted in have an asterisk in the icon
Regular - A thread that isn't an announcement or a sticky
Announcement - a special announcement topic created by an admin or moderator
Sticky - A thread that has been stuck to the top of the forum list because of its importance
Here are the icons and what they mean
A regular, read thread
a regular, read thread that you have posted in
A regular, read thread, but hot
A regular , read thread that you have posted in but hot
A regular, unread thread
A regular, unread thread, that you have posted in
A regular, unread thread, but hot
A regular , unread thread that you have posted in but hot
A read sticky thread
A read sticky thread that you have posted to
An unread sticky thread
An unread sticky thread that you have posted to
A read announcement
A read announcement that you have posted to
An unread announcement
An unread announcement that you have posted to
Threads with unread posts also get an second smaller icon between the Thread Icon and the Thread title to jump to the first unread post in the thread
The overall forum listing uses two icons:
You have no unread topics in the forum
You have at least one unread threads in the forum
The Read/Unread status only applies if you are logged in. If you are not logged in, then all threads and forums get "Read" icons.
and of course, if you are not logged in, nothing gets tagged as "yours" with an asterisk