What is the difference between total utility &marginal utility?

In economics, the marginal utility of a good or of a service is the utility of the specific use to which an agent would put a given increase in that good or service, or of the specific use that would be abandoned in response to a given decrease. In other words, marginal utility is the utility of the marginal use — which, on the umption of economic rationality, would be the least urgent use of the good or service, from the best feasible combination of actions in which its use is included