As U.S. tariffs take effect, the Associated Press explains what tariffs are and how they work

Starting from midnight, imports from Canada and Mexico are 25%, and Canadian energy products are subject to a 10% import tariff. Trump doubled the 10% tariff on Chinese imports to 20% in February, and Beijing retaliated on Tuesday, with tariffs as high as 15% at a wide range of U.S. farm exporters. It also expanded the number of U.S. companies under export control and other restrictions, about twenty-two.