A customs union is a kind of commercial treaty in which the signatory countries establish a common foreign commercial policy in order to be able to eliminate or suppress, in a certain way, the duty between those countries.
The objective that customs unions intend to achieve is to be able to take advantage of the efficiency derived from the international division of labor. In this sense, the consumers of the member countries that are influenced by this treaty will see an increased possibility of being able to buy products without having to pay tariffs for them. On the other hand, producers will also see how the market they direct their production is broader and more complex: there is more supply from competing companies.
If there is no customs union, the countries that exchange products with each other must establish appropriate border controls to allow the entry of products from one country to another. In the event that such a union existed, the controls would not be necessary, so that so many economic and human resources would not have to be spent in establishing these control measures between the countries that decide to exchange products. This is because these member countries will have the same foreign trade policy.
However, concepts must be qualified. In the event that freedom of transit is included production factors (labor and capital), we would be talking about the common market and not about the customs union.
Some examples of customs union, of the best known, is the Unión Europea, since there are no tariffs or trade barriers between the countries that comprise it. It is, therefore, a foreign trade policy common to all member countries.