A tariff is a tax imposed on imported goods and services. Here's what to know about them, why they're used, and what economists say.

US President Donald Trump has rattled economies and markets with punitive tariffs on goods from the nation’s largest trade partners. So far he’s implemented 25% duties on most Canadian and Mexican imports and raised levies on those from China to 20%.

President Donald Trump fulfilled his promise to impose sweeping tariffs on America’s top three trading partners, Mexico, China and Canada.

During his two-hour address to a joint session of Congress, President Trump doubled down on his pledge to impose tariffs on goods imported from China, Mexico and Canada. So, what are tariffs? And who actually has to fork over the money?