Act Utilitarianism holds the normative principle: we should always act according to the Utilitarian Principle. The difference between Act and Rule Utilitarianism is then, whether or not general or universally applicable rules can be derived from the Utilitarian Principle, or whether morality requires us to evaluate each and every individual action in light of the UP.

Read a more about the difference between Act and Rule Utilitarianism from the Internet Encyclopedia of Philosophy