This document discusses oligopolistic market structures and provides examples of cartels. It summarizes that an oligopoly is characterized by a small number of firms producing either homogeneous or heterogeneous products. A cartel is an explicit agreement among firms in an oligopoly to fix prices, output levels, or market shares. Examples of successful cartels mentioned are OPEC and De Beers. The document also discusses collusion, game theory, and provides an example of cellular operators in India engaging in collusive practices.