Suppose you're having a birthday party, and you're making your friends register for it. (You're weird.)
You create a Registration card with a bunch of structure to determine the fields they have to complete. Then, when they submit the registration, you want to receive an email to notify that they've registered.
First you create an Email template called "My Birthday Email" and set it to be delivered to you.
Then you go to "advanced > rules" on the menus of a Registration card and create a new *on create rule and add "My Birthday Email" to the list.
This means that whenever a Registration card is successfully created, you'll be emailed!
The rule can be overridden by rules on narrower Set.
In this case, the interface for, say, an *on update rule is basically a standard Pointer interface, which means you're basically editing a list of cards. In this case, the list of cards is a behavior to be taken with the given event.