美国有线新闻 CNN 特朗普开始重新协商北美自贸协定（在线收听）

And President Trump says he's beginning to renegotiate NAFTA, the North American Free Trade Agreement between the U.S., Canada and Mexico.

REPORTER: Italian fabrics, Japanese Kobe beef, American cars, where there are goods, there's trade.

Trade deals make it easier and cheaper for countries to trade with one another. They're not necessary, but there's a price to pay if you don't have one. Countries generally slap an extra charge on foreign products when they cross the border.

Free trade deals can get rid of these tariffs, pushing prices down and giving customers better access to foreign goods. They can strengthen diplomatic ties. Almost every country in the world has them. Around 230 trade deals went into force between 2000 and 2016, according to World Trade Organization.

So, how do they work?

Let's say two or more countries, Country A, Country B, and Country C, decide they want a deal. They gather important people around the table, lawyers, negotiators, regulators, and they hammer out the details. They'll work out the different sectors they want to focus on and discuss ways to align their industry rules. Leaders give their stamp of approval and generally, lawmakers have to ratify everything.

But it takes time. Modern, large scale trade deals between multiple countries can take years to sign and there's a lot of back and forth. The recent free trade deal between Canada and the European Union took nearly a decade to sign. Even smaller trade deals can often take a year or two.

But not everyone is a fan. Critics say it's better when the tariffs are in place and everyone minds their own business. They say cheap foreign products drown out domestic industry and hurt local jobs. But when trade barriers go up, countries can hurt themselves, leading to higher prices, potential shortages and unhappy customers.