Why do nations trade?

Why do nations trade?  What is meant by the concept of “Comparative Advantage”?  Could a nation be better off economically, if it practiced an isolation policy?The United States has had a significant trade imbalance for several years.  What are the problems associated with having a negative trade balance?  What can be done to correct the imbalance?How are exchange rates determined?  What is the significance of currency devaluations to the home country?  To other countries?