This document discusses key concepts in consumer theory, including utility, cardinal and ordinal utility, marginal utility, total utility, the law of diminishing marginal utility, and the law of equi-marginal utility. Utility refers to the satisfaction derived from consuming a good and is subjective in nature. The law of diminishing marginal utility states that the marginal utility derived from additional units of consumption decreases as consumption increases. The law of equi-marginal utility holds that consumers allocate their budget in a way that equalizes the marginal utility across goods consumed to maximize total utility.