When people use the term utilitarianism they are usually referring to what is called "Act Utilitarianism", which states that the morality of any given action can be derived by taking the sum of the total pleasure produced (quality of pleasure * number of people) and subtracting the total pain.
Rule utilitarianism says that in order to maintain a society where everyone feels safe and comfortable, certain rules must be obeyed without exception. Therefore, given a situation where the action with the greatest immediate utility contradicts one of these rules, the rule must take precedence, for the greatest overall utility.
The difference is subtle, but distinct. Rule utilitarianism has a broader scope in it's analysis of consequence, considering not only immediate effects but also universalization of the act.
Rule utilitarianism is a form of "indirect utilitarianism".
For more, see: Rule Utilitarianism is the Best Form of Utilitarianism