In an oligopoly, there are usually just a small number of companies involved and they will collaborate to control the market and trade. For example, in 2008, there were just four cell phone companies controlling 90% of the cell phone market in the United States (Verizon, AT&T, Sprint, and T-Mobile). If these companies strike a formal agreement to collude on pricing, we would call them a cartel. While cartels are illegal in many places and most instances, OPEC (Organization of Petroleum Exporting Companies) is a legal cartel that was intended to stabilize pricing and availability of oil around the world.
In contrast to an oligopoly is the second form of imperfect competition called monopolistic competition. In this scenario, there are many sellers producing many products. Most products sold here are differentiated by their brand, minor features, and/or quality. This is a very common situation for products sold in the United States. (In the marketplace, of course, nothing is so easily separated, a lot of overlap exists, and there are many arrangements made between individual buyers and sellers.)