forums do update automaticly. You still need to refresh to see if a new post has been made, the forum queries the database each time you load the page...not constantly. If your asking about when you see that there is a new post and you go into the thread and you have to refresh to see it, it's because of your cashe memory.
Aye, your cashe stores recent pages you have visited so that they will load much faster when/if you revisit them soon after. There is a setting in most browsers to turn it off.