Markets can exist anywhere buyers and sellers interact, through various means of communication, to exchange goods and establish a single price. A market structure describes the characteristics of a market that influence firm behavior and performance. The main types of market structures are perfect competition, monopoly, monopolistic competition, and oligopoly. Perfect competition has many small firms and buyers, homogeneous goods, no barriers to entry or exit, and perfect information. A monopoly has a single seller controlling the entire market. Monopolistic competition and oligopoly involve some level of differentiation between close substitutes sold by a few firms.