Market structure refers to the characteristics of a market that determine the behavior of buyers and sellers, the level of competition, and the pricing and production decisions made by firms. The four main types of market structures are:

1. Perfect competition: In a perfectly competitive market, there are many small firms that produce identical products and have no market power to influence the price. Buyers and sellers have perfect information about the market, and there are no barriers to entry or exit.

2. Monopolistic competition: In a monopolistically competitive market, there are many firms that produce differentiated products and have some market power to influence the price. Buyers and sellers have imperfect information about the market, and there are low barriers to entry or exit.

3. Oligopoly: In an oligopolistic market, there are a few large firms that dominate the market and have significant market power to influence the price. Buyers and sellers have imperfect information about the market, and there are high barriers to entry or exit.

4. Monopoly: In a monopolistic market, there is only one firm that produces a unique product and has complete market power to set the price. Buyers and sellers have no information about the market, and there are high barriers to entry or exit.