This document defines different types of market structures: - Perfect competition has many small firms, homogeneous products, free entry and exit, and firms that are price-takers. - Monopoly has a single seller of a unique product without close substitutes that can influence price. - Monopolistic competition features many small differentiated product sellers with some product differentiation and freedom of entry and exit. - Oligopoly has a market dominated by a small number of interdependent firms that closely watch each other's actions.