Oligopoly is a market structure characterized by a small number of firms that control the entire market, influencing pricing and output due to their interdependence. It can be categorized into pure oligopoly (homogeneous products) and differentiated oligopoly (differentiated products), with features including barriers to entry and potential for collusion. The document also discusses various models of oligopoly, including non-collusive and collusive oligopoly, as well as game theory applications that explain strategic interactions among firms.