Why do nations trade, and what barriers can restrict international trade?

"Nations trade because they gain by doing so. The principle of comparative advantage states that each country should specialize in the goods it can produce most readily and cheaply and trade them for those that other countries can produce most readily and cheaply. The result is more goods at lower prices than if each country produced by itself everything it needed. Free trade allows trade among nations without government restrictions. The three major barriers to international trade are natural barriers, such as distance and language; tariff barriers, or taxes on imported goods; and nontariff barriers. The nontariff barriers to trade include import quotas, embargoes, buy-national regulations, and exchange controls. The main argument against tariffs is that they discourage free trade and keep the principle of comparative advantage from working efficiently. The main argument for using tariffs is that they help protect domestic companies, industries, and workers. A tariff is a tax imposed on imported goods. Protective tariffs are tariffs that are imposed in order to make imports less attractive to buyers than domestic products. Protectionism is the policy of protecting home industries from outside competition by establishing artificial barriers such as tariffs and quotas. A preferential tariff is a tariff that is lower for some nations than for others."