This exact same question was asked on Yahoo Answers about a month ago so I am just reanswering an already asked question somewhere else. "In finance, the exchange rate (also known as the foreign-exchange rate, forex rate or FX rate) between two currencies specifies how much one currency is worth in terms of the other. For example an exchange rate of 123 Japanese yen (JPY, ¥) to the United States dollar (USD, $) means that JPY 123 is worth the same as USD 1. The foreign exchange market is one of the largest markets in the world. By some estimates, about 2 trillion USD worth of currency changes hands every day.The spot exchange rate refers to the current exchange rate. The forward exchange rate refers to an exchange rate that is quoted and traded today but for delivery and payment on a specific future date. A market based exchange rate will change whenever the values of either of the two component currencies change. A currency will tend to become more valuable whenever demand for it is greater than the available supply. It will become less valuable whenever demand is less than available supply (this does not mean people no longer want money, it just means they prefer holding their wealth in some other form, possibly another currency). Increased demand for a currency is due to either an increased transaction demand for money, or an increased speculative demand for money. The transaction demand for money is highly correlated to the country's level of business activity, gross domestic product (GDP), and employment levels. The more people there are out of work, the less the public as a whole will spend on goods and services. Central banks typically have little difficulty adjusting the available money supply to accommodate changes in the demand for money due to business transactions. The speculative demand for money is much harder for a central bank to accommodate but they try to do this by adjusting interest rates. An investor may choose to buy a currency if the return (that is the interest rate) is high enough. The higher a country's interest rates, the greater the demand for that currency. It has been argued that currency speculation can undermine real economic growth, in particular since large currency speculators may deliberately create downward pressure on a currency in order to force that central bank to sell their currency to keep it stable (once this happens, the speculator can buy the currency back from the bank at a lower price, close out their position, and thereby take a profit). In choosing what type of asset to hold, people are also concerned that the asset will retain its value in the future. Most people will not be interested in a currency if they think it will devalue. A currency will tend to lose value, relative to other currencies, if the country's level of inflation is relatively higher, if the country's level of output is expected to decline, or if a country is troubled by political uncertainty. For example, when Russian President Vladimir Putin dismissed his Government on February 24, 2004, the price of the ruble dropped. When China announced plans for its first manned space mission, synthetic futures on Chinese yuan jumped (since China's currency is officially pegged, synthetic markets have emerged that can behave as if the yuan were floating. The foreign exchange markets are usually highly liquid as the world's main international banks provide a market around-the-clock. The Bank for International Settlements reported that global foreign exchange market turnover daily averages in April was $650 billion in 1998 (at constant exchange rates) and increased to $1.9 trillion in 2004 (Triennial Central Bank Survey of Foreign Exchange and Derivatives Market Activity 2004 - Final Results). The biggest foreign exchange trading centre is London, followed by New York and Tokyo. For converting from ome currency do not calculate: just visit Yahoo Finance" Source: http://ph.answers.yahoo.com/question/index?qid=20070818114555AAytYqv

Currency rates are determined largely by market forces. The amount of exports and imports of a country is one factor, as well as how the currency is being traded on the foreign exchange market. Some countries, like China, set the rate artificially and do not allow market forces to determine its currency's value. Until 2005, China fixed its exchange rate at 8.28 renminbi per $1US. Since 2005, China has allowed the value to "float" in a small band with respect to a number of foreign currencies such as the U.S. dollar, Euro, Japanese Yen and others.

"A market based exchange rate will change whenever the values of either of the two component currencies change. A currency will tend to become more valuable whenever demand for it is greater than the available supply. It will become less valuable whenever demand is less than available supply (this does not mean people no longer want money, it just means they prefer holding their wealth in some other form, possibly another currency). Increased demand for a currency is due to either an increased transaction demand for money, or an increased speculative demand for money. The transaction demand for money is highly correlated to the country's level of business activity, gross domestic product (GDP), and employment levels. The more people there are out of work, the less the public as a whole will spend on goods and services. Central banks typically have little difficulty adjusting the available money supply to accommodate changes in the demand for money due to business transactions." Source and further information: http://en.wikipedia.org/wiki/Exchange_rate