Batra, Ravi, “The Myth of Free Trade, Touchstone, The Pooring of America,” New York, NY, 1993

“Although tariffs on imports have been the principal instrument for regulating trade, protection can be granted in many ways.  The idea is to raise the price of imported goods or to reduce the cost of production for domestic import-competing industries.
Tariffs are simply taxes imposed on foreign products at the port of entry.  They are mainly of two types.  An ad valorem, or value tariff is a certain percentage, normally fixed, by which the product is raised.” p. 178-179