"The most common
barrier to trade is a tariff—a tax on imports. Tariffs raise the price of imported goods relative to domestic goods (goods produced at home).
Another common
barrier to trade is a government subsidy to a particular domestic industry. Subsidies make those goods cheaper to produce than in foreign markets. This results in a lower domestic price. Both tariffs and subsidies raise the price of foreign goods relative to domestic goods, which reduces imports.
Yet another
barrier to trade is an embargo—a blockade or political agreement that limits a foreign country's ability to export or import.
Barriers to trade are often called 'protection' because their stated purpose is to shield or advance particular industries or segments of an economy. From an economic perspective, though, the costs to the economy almost always outweigh the benefits enjoyed by those who are protected."
(Library Economics and Liberty,
Barriers to Trade, visited 2010-04-13)