People may become confused between ACT and RULE utilitarianism in the beginning because they seem to be the same. The more that you understand their differences, the more that you will know how different they are. When you say act utilitarianism, this means that you are only looking into the results of something that has been done.
The act may have been something that is very simple. When you say RULE utilitarianism, this means that you would like to make sure that there are various consequences that are available after doing a rule of conduct.
For example, for those who follow rule utilitarianism, they would say that murder is wrong not only because of the act itself but because of how much it can affect the happiness of everyone involved. When you say act utilitarianism, the act itself is being condemned because it is wrong.