If in an oligopoly market, the firms compete with each other, it
is called a non-collusive, or non-cooperative oligopoly. If the
firm cooperate with each other in determining price or output or
both, it is called collusive oligopoly, or cooperative
oligopoly.
Collusive oligopoly exists when the firms in an Oligopolistic
market charge the same prices for their products, in affect acting
as a monopoly but dividing any profits that they make.
Non collusive oligopoly exists when the firms in an oligopoly do
not collude and so have to be very aware of the reactions of other
firms when making price decisions.