Mercosur is the biggest trading bloc in South America. It was founded back in 1991 by the Treaty of Asuncion. Like most trading blocs, its purpose is to promote free trade among its members. It tries to do this in a variety of ways, including attempts to reduce tariffs.
There are four main members of Mercosur. They are Argentina, Brazil, Paraguay and Uruguay. Venezuela is trying to join but has so far not been approved by Brazil and Uruguay.
Another trading bloc in South America (that will make Mercosur obsolete if it is fully implemented) is UNASUR, which would include 12 South American countries.