President Donald Trump unveiled a trade agreement with the European Union on Sunday, making it the latest in a series of accords as the White House threatens to slap tariffs on dozens of countries this week.

Tariffs are taxes charged on goods bought from other countries. Typically, they are a percentage of a product's value. So, a 15% tariff means that a $100 product imported to the US from the EU will have a $15 dollar tax added on top - taking the total cost to the importer to $115.

Both sides hailed the agreement as the biggest ever. But it will come at a cost to the European Union, and many details have yet to be nailed down.