Demand Curve: The demand curve shows the relationship between price and quantity demanded for a particular good or service. It typically slopes downward because as prices decrease, quantity demanded increases.
Marginal Utility: Marginal utility refers to the additional satisfaction or benefit gained from consuming one more unit of a good or service. Initially, marginal utility tends to be high but diminishes as more units are consumed.
Production Possibilities Frontier (PPF): The PPF represents all possible combinations of two goods that an economy can produce given its resources and technology. It is usually depicted as a curve that is upward sloping due to increasing opportunity costs.