the "view active topics"and "view new posts" - should show you the latest threads that have been posted on since you last logged in (i think you need to be logged in to view these though) - it's just a quick way to see the most recent activity on the forum - also, at the bottom of the active threads page - there is a drop down box which is set at a default of 7 days - just select another time frame (say 1 month) and it will show you all active topics for the last 4 weeks
if there are particular threads that you are interested in following - you can subscibe to them - and you'll be notified each time a post has been made on that thread