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Trade barriers may occur in international trade when goods have to cross political boundaries. A trade barrier is a restriction on what would otherwise be free trade. The most common form of trade barriers are tariffs, or duties (the two words are often used interchangeably in the context of international trade), which are usually imposed on imports. There is also a category of nontariff barriers, also known as nontariff measures, which also serve to restrict global trade.

There are several different types of duties or tariffs. An export duty is a tax levied on goods leaving a country, while an import duty is charged on goods entering a country. A duty or tariff may be categorized according to how it is calculated. An ad valorem tariff is one that is...