Oligopoly

In economics, an oligopoly is a market form in which the market or industry is controlled by a small number of sellers. Usually, the market has high barriers to entry, which prevents new firms from entering the market or even be able to have a significant market share. 

As there are only a few sellers in the market, each seller would take note of the actions made by one another, and thinks about how the other sellers will respond when making decisions. As such, there is a possibility in which an oligopoly can come together to make a common decision that allows them to have less competition and charge higher prices for consumers. 

Examples
In many countries, some country-held companies were privatized. Very often, this privatization lead to oligopolies. In many countries, there are only a handful of companies providing networks for mobile phones. They control the prices for accessing the network. That is why using a mobile phone is often much more expensive than using a land line one.

Trains run by private sectors are much costlier than those run by a government. As a government gives rights to private sectors to get a hold of some other sectors, they take advantage of it.

 Related pages 
Oligarchy
Plutocracy

 Other websites 
 Microeconomics by Elmer G. Wiens: Online Interactive Models of Oligopoly, Differentiated Oligopoly, and Monopolistic Competition
 Vives, X. (1999). Oligopoly pricing, MIT Press, Cambridge MA. (A comprehensive work on oligopoly theory)