Why is global trade important to the United States, and how is it measured?

"International trade improves relations with friends and allies, eases tensions among nations, helps bolster economies, raises people’s standard of living, and improves the quality of life. The United States is still the largest importer and exporter in the world. We export a fifth of our industrial production and about a third of our farm crops. Two concepts important to global trade are the balance of trade (the difference in value between a country’s exports and its imports over some period) and the balance of payments (the difference between a country’s total payments to other countries and its total receipts from other countries). The United States now has both a negative balance of trade and a negative balance of payments. Another important concept is the exchange rate, which is the price of one country’s currency in terms of another country’s currency. Currencies float up and down based upon the supply of and demand for each currency. Sometimes a government steps in and devalues its currency relative to the currencies of other countries."