Utilitarianism is a normative moral theory, i.e. a way of deciding what actions are morally good.
It is form of consequentialism, meaning that what is considered of moral importance is only the consequences of actions. An action is good if it has good consequences, no matter what the intention was behind it. In this sidebar I will only look at hedonistic utilitarianism that considers pleasure good and pain bad and therefore strives to maximize pleasure. There are several objections to using this the basis of morality. Utilitarianism is not concerned with persons.