Your "Subscriptions" notify you when content is added or updated in your areas of interest. You control your subscriptions on the My Account pages. Though powerful, this is a bit complicated...
Home >> My account >> Subscriptions >> Overview
Auto-subscribe : Select all three auto-subscribe boxes, if you want to know when people respond to you.
Notify poster of own posts : Maybe, if you want to see your comments in order with others'. (I say yes.)
Digest mode : Definitely Yes! You do not want individual emails for each change!!!
Send interval : If you set this to "as soon as possible", your digest is actually mailed the next time the hourly housekeeping job runs - so in effect, at the moment, values "as soon as possible", "every 15 minutes", and "hourly" all have the same effect - hourly. The higher values (3 hours, or daily) work as advertised, though. And if you select a lower value, you'll get mails more often sometimes, or if I change the housekeeping schedule.
On updates : Yes.
On comments : Definitely Yes! Note that any reply to a forum topic, is a "comment".
Home >> My account >> Subscriptions >> Pages
The purpose of this interface is to unsubscribe from things you've already subscribed to via auto-subscribe or one of the other Subscriptions tab. So uncheck the ones you don't want to hear from anymore, and click the Save button.
Home >> My account >> Subscriptions >> Blogs
You can subscribe / unsubscribe to personal blogs here. If you want to subscribe to all blogs, do it on the next tab.
Home >> My account >> Subscriptions >> Content Types
Subscribe / unsubscribe by generic page type (Blog entry, Forum topic, Book page, Story).
Home >> My account >> Subscriptions >> Categories
This lists whole forums (can subscribe / unsubscribe to whole categories of forum topics), and tags. Most content can be "tagged" with its topic (tomato, lettuce, hardware, drupal, etc.) Subscribing to a tag here, allows you to get email updates whenever there's new content of a particular tag (tomato discussion, etc.)
But also, more importantly, this is the only way at this time to get a listing of all content tagged "tomato". (Click on the word "tomato".) The tag-content listings also provide an RSS feed link at the bottom, so you can get an RSS feed on that topic, as well as an email subscription.
However, this all depends on others tagging their content.