People also have taken to using them to make quick comments when they don't feel a completely seperate post is necessary. ;-)
Kinda like this hash tag thing.
To use tags, you add words or phrases to threads to help describe the content. For instance, if the subject matter is 'PlayStation Home Estates' then you can add the tag 'PlayStation Home Estates' to the tag list. But you could also add tags like 'PSHome Estate', and '' (depending, of course, on the nature of the thread).
This will categorize this thread with all other threads that have matching tags, whether or not they have the word 'Home Estates' in them.