Market structure refers to the characteristics of a market, including the number of firms, ease of entry/exit of firms, and degree of product differentiation. The main market structures are perfect competition, monopolistic competition, oligopoly, and monopoly. Perfect competition is characterized by many firms producing homogeneous products, freedom of entry and exit, and firms being price takers. Monopolistic competition has many firms producing differentiated products and easy entry. Oligopoly has a few large firms producing differentiated or homogeneous products, with interdependence and barriers to entry. Monopoly is a single seller of a product with no close substitutes and the firm being a price maker.