Monopolistic Competition and Oligopoly:

Perfect competition and monopoly have been described as two extreme kinds of market structure that are rarely found in practice. Monopolistic competition and oligopoly lie between these two extremes reflecting the industries of most firms in the real world.

Monopolistic competition describes a market or industry where there are large numbers of firms producing (selling) products which consumers believe are close but not perfect substitutes for each other.

On the other hand, Oligopoly refers to an industry or market with a few firm producing (selling) products that are either homogeneous or differentiated.The firms are so few that the action of one firm affects the decision of other firms in the industry.Where the firms produce homogeneous products, it is called pure or perfect Oligopoly and where they produce heterogeneous or differentiated products it is called imperfect or differentiated oligopoly. Examples of oligopoly are found in the few industries producing automobiles, cement, cigarettes, rubber tyres, newspapers and so on.
 
Duopoly is a special case of the theory of oligopoly. Duopoly is a market structure in which only two firms constitute the industry - i.e. they produce the entire market supply. A change in the price - output policy of one will affect the other, and may elicit a chain of reactions.