The document discusses different market structures including perfect competition, monopoly, monopolistic competition, oligopoly, and duopoly. It describes the key characteristics of each structure such as the number and size of firms, level of control over price, barriers to entry/exit, product homogeneity, and profit levels. More competitive structures have many small firms, undifferentiated products, free entry/exit while less competitive structures have few dominant firms, differentiated products, and high barriers to entry.