On some SE sites, there is a tab called "Subscriptions" with a title of "Filter Subscriptions". What is this, and how does one use it? Example.
I did find a link that clears it up a bit: http://stackexchange.com/filters
It lists all the subscriptions to tags you have.
When you hover a tag shown under a question, you see a popup similar to the following one.
When you click on subscribe, you get emails about new questions tagged with that tag, and probably about new answers to those questions.