Perfect competition
Perfect competition a market structure characterized by a large number of firms so small relative to the overall size of the market, such that no single firm can affect the market price or quantity exchanged. Perfectly competitive firms are price takers. They set a production level based on the price determined in the market. If the market price changes, then the firm re-evaluates its production decision. This means that the short-run marginal cost curve of the firm is its short-run supply curve. The four characteristics of perfect competition are: (1) large number of small firms, (2) identical products, (3) perfect resource mobility, and (4) perfect knowledge.
Think of the number of dry cleaners in your town - mine has at least 8.
Oligopoly
Oligopoly is a market structure characterized by a small number of relatively large firms that dominate an industry. The market can be dominated by as few as two firms or as many as twenty, and still be considered oligopoly. With fewer than two firms, the industry is monopoly. As the number of firms increase (but with no exact number) oligopoly becomes monopolistic competition.
Because an oligopolistic firm is relatively large compared to the overall market, it has a substantial degree of market control. It does not have the total control over the supply side as exhibited by monopoly, but its capital is significantly greater than that of a monopolistically competitive firm. An example might be the security ratings agencies of S&P and Moodys. While there are other market players, these two dominate.
Monopoly
A monopoly is a market structure in which there is only one producer/seller for a product. In other words, the single business is the industry. Entry into such a market is restricted due to high costs or other impediments, which may be economic, social or political. For instance, a government can create a monopoly over an industry that it wants to control, such as electricity. Another reason for the barriers against entry into a monopolistic industry is that oftentimes, one entity has the exclusive rights to a natural resource. For example, in Saudi Arabia the government has sole control over the oil industry. A monopoly may also form when a company has a copyright or patent that prevents others from entering the market. Pfizer, for instance, had a patent on Viagra.
Monopolistic competition
Monopolistic competition is a market structure characterized by a large number of relatively small firms. While the goods produced by the firms in the industry are similar, slight differences often exist. As such, firms operating in monopolistic competition are extremely competitive but each has a small degree of market control. The four characteristics of monopolistic competition are: (1) large number of small firms, (2) similar, but not identical products, (3) relatively good, but not perfect resource mobility, and (4) extensive, but not perfect knowledge. Think major grocery store chains in your town.
