Explain the difference between act utilitarianism and rule utilitarianism?

The difference between act utilitarianism and rule utilitarianism:
Utilitarianism's guiding principle is to increase happiness or 'utility' as explained by John Stuart Mill. Act utilitarianism - for each action, we consider the impact on utility or happiness and judge the action by its 'net impact'. With Act the choice is made based on the best possible consequence. For example telling someone a non truth because the real truth might cause them to be unhappy and that could have a domino effect. Rule utilitarianism follows 'moral rules' where there are no exceptions. The only rule is 'Utility'. Every act is evaluated according to the utility. Does it or does it or doesn't it produce Happiness?
Both ACT and RULE utilitarians must assume nothing.
ACT Utilitarian measures the consequences of one single ACT.
RULE Utilitarian measures the consequences of the act repeated over and over again through time as if it were to be followed as an actual RULE whenever similar circumstances arise.