Utilitarianism is an ethical theory that says an action is morally right if it benefits the greatest number of beings with the greatest good. You determine what is right by calculating the amount of pleasure or suffering you think your actions may cause, then the right action will be the one that gives most pleasure or least suffering to the majority concerned. Say some of your dinner guests are vegetarians and you wonder whether to serve roast pig. As a strict utilitarian you poll your guests beforehand. Five guests say no to pig and twenty-five guests say yes. Therefore you serve pig and bring pleasure to the majority of guests, hard luck for the pig and the six veggies.