This document summarizes different market structures: - Perfect competition has many small firms, identical products, free entry and exit, and firms are price takers. - Monopoly has a single seller, high barriers to entry, and the firm can influence the market price. - Monopolistic competition is between perfect competition and monopoly with many firms selling differentiated goods. - Oligopoly has a few large firms producing either differentiated or similar products, where each firm is influenced by competitors.