"A tariff is a customs duty levied on imported and exported goods and services. Historically, countries used tariffs as a primary means of collecting revenue. Today, other taxes account for most government revenue in developed countries. Tariffs are now typically used to protect domestic industries or as leverage in trade negotiations and disputes. The U.S. Constitution empowers Congress to set tariffs, a power that it has partially delegated to the President. The United States is also a member of the World Trade Organization (WTO) and a party to 14 free trade agreements (FTAs), which include specific tariff-related commitments. Congress and the President thus create U.S. tariff policy within the context of a rules-based global trading system."
CRS In Focus, IF11030
Congressional Research Service: https://crsreports.congress.gov/