Monopoly- the exclusive possession or control of the supply or trade in a commodity or service.Characteristics Profit Maximizer: Maximizes profits. Price Maker: Decides the price of the good or product to be sold, but does so by determining the quantity in order to demand the price desired by the firm. High Barriers: Other sellers are unable to enter the market of the monopoly. Single seller: In a monopoly, there is one seller of the good that produces all the output.Therefore, the whole market is being served by a single company, and for practical purposes, the company is the same as the industry.

oligopoly- is a market form in which a market or industry is dominated by a small number of sellers (oligopolists).Oligopoliescan result from various forms of collusion which reduce competition and lead to higher prices for consumers.

Profit maximization conditionsAn oligopolymaximizes profits.Ability to set priceOligopolies areprice settersrather than price takers.[2]Entry and exitBarriers to entry are high.[3]The most important barriers are government licenses, economies of scale, patents, access to expensive and complex technology, and strategic actions by incumbent firms designed to discourage or destroy nascent firms. Additional sources of barriers to entry often result from government regulation favoring existing firms making it difficult for new firms to enter the market.[4]Number of firms"Few" a "handful" of sellers.[3]There are so few firms that the actions of one firm can influence the actions of the other firms.[5]Long run profitsOligopolies can retain long run abnormal profits. High barriers of entry prevent sideline firms from entering market to capture excess profits.Product differentiationProduct may be homogeneous (steel) or differentiated (automobiles).[4]

Monopolistic competitionis a type of imperfect competitionsuch that many producers sell products that are differentiated from one another (e.g. by branding or quality) and hence are not perfect substitutes.

Duopoly - a situation in which two suppliers dominate the market for a commodity or service.

Monopsony is amarket formin which only one buyer interfaces with would-be sellers of a particular product.