An oligopoly (from olígos meaning “few” and polein meaning “to sell”) is a market form in which a market or industry is dominated by a small number of sellers . Oligopolies can result from various forms of collusion which reduce competition and lead to higher prices for consumers.

With few sellers, each oligopolist is likely to be aware of the actions of the others. According to game theory, the decisions of one firm therefore influence and are influenced by the decisions of other firms.

It is kind of controlled Monopoly.

Examples: The petroleum and gas industry is dominated by Indian Oil, Bharat Petroleum, Hindustan Petroleum, Reliance And TATA

source: http://en.wikipedia.org/wiki/Oligopoly