What are the four types of market structure and what defines perfect competition?

"One of the characteristics of a free-market system is that suppliers have the right to compete with one another. The number of suppliers in a market defines the market structure. Economists identify four types of market structures: (1) perfect competition, (2) pure monopoly, (3) monopolistic competition, and (4) oligopoly. Table 1.3 summarizes the characteristics of each of these market structures.\n\nCharacteristics of perfect (pure) competition include:\n\n• A large number of small firms are in the market.\n\n• The firms sell similar products; that is, each firm’s product is very much like the products sold by other firms in the market.\n\n• Buyers and sellers in the market have good information about prices, sources of supply, and so on.\n\n• It is easy to open a new business or close an existing one.\n\nIn a perfectly competitive market, firms sell their products at prices determined solely by forces beyond their control. Because the products are very similar and each firm contributes only a small amount to the total quantity supplied by the industry, price is determined by supply and demand. A firm that raised its price even a little above the going rate would lose customers. In the wheat market, for example, the product is essentially the same from one wheat producer to the next. Thus, none of the producers has control over the price of wheat.\n\nPerfect competition is an ideal. No industry shows all its characteristics, but the stock market and some agricultural markets, such as those for wheat and corn, come closest. Farmers, for example, can sell all of their crops through national commodity exchanges at the current market price."