Utility is a generic term used to specify how much a certain action gives results according to an agent’s preferences. Its unit – util or utilon – is an abstract arbitrary measure that assumes a concrete value only when the agent’s preferences have been determined through an utility function.
It’s a concept rooting from economics and game theory, where it measures how much a certain commodity increases welfare. One of the clearest examples is money: the price that a person is willing to pay for something can be considered a measure of the strength of his preference for it. Thus, a willingness to pay a high sum for something implies that the person has a strong desire for it, i.e. it has a high utility for him. Although it has been argued that utility is hard to quantify when dealing with human agents, it is widely used when designing an AI capable of planning.
Utilitarianism is a moral philosophy advocating actions which bring the greatest welfare for the greatest amount of agents (generally humans) involved.