Utilitarianism has remained influential and vibrant within ethical canon since Mill's treatise was first published in 1861. As time has passed, however, the term has evolved to the point where "utilitarianism" has become an umbrella term for multiple theories that engage the Greatest Happiness principle in different ways. Two such theories are called "act utilitarianism" and "rule utilitarianism."
A person operating under act utilitarianism applies the greatest happiness principle in considering the immediate consequences of actions. One could say that the ethical universe in which they operate is contained to those engaged in an action as well as those impacted by it. This is the framework that is easiest to extract from Mill's work on a first reading.
Rule utilitarianism, in contrast, takes a broader view of the ethical universe. In applying the greatest happiness principle to actions and situations, it considers the implications for utility if the action were taken to establish a rule. Thus, an action that might be sanctioned by act utilitarianism because of its immediate impacts could be impermissible by the standards of a rule utilitarian.
Consider, as an example, the classic ethical dilemma of an out-of-control trolley. There are five people on the track ahead of it, about to be killed, and you are standing off to the side with another bystander. The only way to save the five people is to push the person next to you in front of the train. What do you do?
The prototypical act utilitarian would push the bystander, because five lives are greater than one. In contrast, the prototypical rule utilitarian would not push the bystander, because such an action would effectively endorse murder, which would produce far more lasting pain than the pain of the death of five people. So we see that the two ethical views, while operating on the same basic principle, draw opposing ethical conclusions based on the ways in which that principle is applied.
Two further points are worth noting about rule utilitarianism. The first is that, while utilitarianism is touted by many as the paragon of consequentialist ethics, rule utilitarianism looks a lot like a deontological ethical stands inasmuch as it universalizes the action under analysis in order to judge its ethical quality. This method is similar to Kant's categorical imperative; yet we must be aware that this is a seeming similarity only: Kant's categorical imperative turns on the a priori concern of logical consistency, whereas the universalization that rule utilitarianism employs is still ultimately concerned with a posteriori consequences of actions.
The second point is that, though act utilitarianism is easier to grasp on a first reading, it is more likely that Mill had rule utilitarianism in mind in his treatise. Although he never explicates this, there is evidence to this effect: for example, his only apparent critique of Kant is the fact that Kant does not admit that his categorical imperative's validity rests on utility. A categorical imperative borne of utility, as we have just said, is essentially rule utilitarianism.