What is an oligopoly?

Oligopoly is a market situation in which there are few sellers competing against each other, using price/product differentiation as their tools to gain marketshare .
The main attribute of oligopoly firms is that, similar to monopoly situations, the ability to enter the market (telecoms or motor manufacturing) is difficult (barriers to entry), and incumbents (those already in the market) employ strategies which seek to win marketshare (i.e. they are not only profit driven) from each other, create large economic profits.
Some oligopolistic markets even demonstrate collusion (cooperation amongst competing firms), which is another strategy employed by incumbents to keep new players out.