Well, read the FAQ
If you post a new thread, or reply to an existing thread, you can choose to be notified by email when someone else replies to that thread. You can also choose to receive email notifications of new posts in a thread without posting in that thread, by clicking the 'Subscribe to this thread' link displayed in the thread page.
If you choose to stop receiving email notifications from a thread, you can turn off the option by either editing your post, or by unsubscribing from the thread here.
Only registered users may receive email notification, and they can set whether they want to receive email notification by default by editing their options.
Email notification is also referred to as 'Thread Subscription'.