You have a lot of control as to how you are notified when threads are updated.
Under your Profile (Go under Profile, then Edit Profile), you will see a Notifications tab. This gives you the ability to control what forum actions you are notified of, and how you are notified. I'll highlight a couple:
Participated - this is how you are notified if a thread you have participated in has been updated.
Bookmark - you can use this to update you on threads you are following, but haven't participated in (see below)
To bookmark a thread, you have to be at the view that shows you all the threads within a board (you can be viewing the individual thread). Next to each thread is a check box. Check the box next to the thread you want to bookmark, then under the Actions dropdown, select Bookmark.