You have probably heard someone justify their actions as being for the greater good. Utilitarianism is the ethical theory behind such justifications.
Utilitarianism is a teleological theory of ethics. Teleological theories of ethics look at the consequences – the results of an action – to decide whether it is right or wrong. Utilitarianism is a consequentialist theory. It is the opposite of deontological ethical theories that are based on moral rules, on whether the action itself is right or wrong.