Welcome!
tags are what help make searching for content easier. say you wanted to find a thread on a Tein S Tech Spring, someone in the past couldve tagged their thread with words like "tein" or "s tech" etc, to help you when you search later on. Its alot like a keyword. Not too many people tag stuff here though, as far as i see.
to know when you get a thread reply, you could just check back every now and then, but if you look above the first post, youll see a drop down menu that says "Thread Tools" and in there is an option to subscribe to the thread, which will send you an email to your registered email address whenever someone posts in the thread.
also, when your looking at the forum, pay attention to the little envelope icons to the left of a thread listing. It can tell you if there are no new posts, or new posts as well
does that help?