Setting (or Rule Types) are options about how a card looks and behaves.
A Rule applies a specific Setting to a specific Set of cards. For example, the *update setting governs who can update cards. So one could make a rule with that setting that says only Administrators can create HTML cards.
See the rules card to learn more about how to use Decko Rules.
Note: the Setting+howto video is badly out of date and thus removed from this page. We hope to update it soon.