This document discusses different market structures: perfect competition, imperfect competition (monopolistic competition, oligopoly, monopoly). Perfect competition has many small buyers and sellers of homogeneous products with perfect information. Imperfect competition has some but not all features of perfect competition, such as product differentiation. Monopolistic competition has many firms selling differentiated products. Oligopoly has a few dominant firms selling homogeneous or differentiated products. Monopoly has a single seller of unique products without close substitutes. The document outlines key characteristics of each market structure and how they impact prices, competition, and market outcomes compared to perfect competition.