When you "subscribe" to a forum, you are choosing to receive all posts to that forum via email, in either a daily digest format (one email containing all posts from forums that you are subscribed to every 24 hours) or as individual emails (one email per post).  All forums are set up to automatically subscribe you to the forum.  However, only some forums will allow you to unsubscribe.  You can also subscribe/unsubscribe to individual discussions in some forums.