A cartel is a group of businesses who cooperate to limit competition. They may set prices or market share, or allocate customers or territories. A cartel is similar to a monopoly, but a monopoly is dominance by a single entity. Both are illegal under antitrust laws in the US and under competition laws in Europe.
In modern times the best known cartels are OPEC, The Organization of Petroleum Exporting Countries, the global oil cartel, and DeBeers, the diamond cartel. Both are able to function by keeping operations out of countries with strong laws.
In the late nineteenth century, cartels were common, especially in international trade.