You can receive an e-mail notification whenever somebody creates a new topic or replies to an existing topic. There are two different kind of subscriptions:
Forum subscription
You receive an e-mail when a new topic is created in a forum. You do not receive any notifications for topics that you have not subscribed to explicitly (see below).
Example: If you want to get a notification whenever a new topic is created in the “Jobs” forum, you should subscribe by clicking the “Subscribe” link below the title (see screenshot). You will not receive notifications for replies to existing topics, unless you subscribe to them (see below).
Topic subscription
You receive an e-mail when somebody posts a reply to a particular topic.
Example: If you want to get an e-mail every time somebody posts in the “Bugs and feature requests” topic in the “General discussion” forum, you should subscribe to it by clicking the “Subscribe” link in the header of the first post (see screenshot).
You can manage (i.e. review and delete) your subscriptions on your profile page.