Economists classify markets by the number of firms, the degree of product differentiation, and the ease with which new firms can enter. At one extreme, perfect competition features many buyers and sellers trading an identical product with free entry and exit and perfect information. In such markets individual firms are price takers; they can sell as much as they wish at the going price but cannot influence it. In the long run, free entry drives economic profit to zero as successful innovations are quickly imitated.
At the opposite extreme, a monopoly consists of a single seller protected by high barriers to entry, giving the firm considerable power to set prices above marginal cost. The result is reduced output and higher prices compared to competitive markets, often prompting government regulation. An oligopoly falls between these poles, dominated by a few large, interdependent sellers whose decisions affect one another. This interdependence frequently leads to strategic behavior, including tacit collusion, price leadership, or outright price wars, making oligopoly outcomes among the most complex to predict.
Monopolistic competition blends elements of both extremes. Many firms sell differentiated products, each enjoying some price-setting power from branding, quality, or location. Short-run economic profits are possible, but free entry erodes them over the long run as new entrants imitate successful features. Firms in monopolistic competition typically operate with excess capacity, producing less than the cost-minimizing scale because differentiation, not cost minimization, is the chief competitive tool.