To get email about changes to something you're looking at, hover your mouse over the gray bar at the bottom of the card and click on the "watch" link that will appear in the right corner.
You can also choose to watch all cards of a given type. For example, if you want to get email whenever any pattern is edited, go to Patterns and go to the gray bar and click on "watch Pattern cards". You could do the same thing with Users.
For a more technical explanation, see http://wagn.org/Notification
.