Utilitarianism is an ethical theory that determines right from wrong by focusing on outcomes. It is a form of consequentialism.
Utilitarianism holds that the most ethical choice is the one that will produce the greatest good for the greatest number. It is the only moral framework that can be used to justify military force or war. It is also the most common approach to moral reasoning used in business because of the way in which it accounts for costs and benefits.
However, because we cannot predict the future, it’s difficult to know with certainty whether the consequences of our actions will be good or bad. This is one of the limitations of utilitarianism.
Utilitarianism also has trouble accounting for values such as justice and individual rights. For example, assume a hospital has four people whose lives depend upon receiving organ transplants: a heart, lungs, a kidney, and a liver. If a healthy person wanders into the hospital, his organs could be harvested to save four lives at the expense of one life. This would arguably produce the greatest good for the greatest number. But few would consider it an acceptable course of action, let alone the most ethical one.
So, although utilitarianism is arguably the most reason-based approach to determining right and wrong, it has obvious limitations.