A customs union is an agreement between countries to eliminate tariffs and other trade barriers on goods traded among them. This allows for free trade within the union, making it easier and cheaper for member countries to exchange products. Additionally, customs unions establish a common external tariff on goods imported from non-member countries, ensuring that all members apply the same trade rules. By joining a customs union, countries can enhance economic cooperation and increase market access for their businesses. Examples of customs unions include the European Union and the Southern Common Market (MERCOSUR), which facilitate trade among their member states while maintaining a unified trade policy with the rest of the world.