The thread with the most recent post is at the top. There are "sticky" threads right above them, but there are usually only 3-4 of those, unless anyone went poll crazy in that forum.
This -
or this -
(popular topic) indicate the latest posts since you last logged into a particular computer.
This -
or this -
(popular topic) indicate the topics that haven't been updated since you last logged into a particular computer.
The "sticky" threads have a flag -
next to them meaning they stay on top until a mod unfeatures them.