Chapter 19 is about international trade, and what the benefits of international trade are.

US Trade Patterns
Imports are the goods and services that we buy from people and firms in other countries. Imports represent 18% of total GDP.
Exports are the goods and services that we sell to people, firms, and governments in other countries. The U.S is the world's largest exporter, but exports represent a small fraction of total output.
The trade balance is the difference between the value of exports and imports. When imports exceed exports we have a negative trade balance called a trade deficit. The opposite is a trade surplus.

Motivation to Trade
Specialization increases total output. Without imports a country cannot consume more than it produces. Because just by changing the mix of output in at country total world output increases, that country has a motivation to trade. This works because when nations specialize in production, they can export one good and import another and end up with more total goods to consume than they had without trade.

PURSUIT OF COMPARATIVE ADVANTAGE
The ability of a country to produce a specific good at a lower opportunity cost than its trading partners is called comparative advantage. Nations export goods with relatively low opportunity costs, and import goods with relatively high opportunity costs. Some countries may have absolute advantage, that is, the ability of to produce a specific good with fewer resources than other countries.

TERMS OF TRADE
Terms of trade is the rate at which goods are exchanged. A country won’t trade unless the terms of trade are superior to domestic opportunities. The terms of trade depend on the willingness of market participants to buy or sell at various prices.

PROTECTIONIST PRESSURES
Workers and producers who compete with imported products have an economic interest in restricting trade