Exchange rates float freely against one another, which means they are in constant fluctuation. Currency valuations are determined by the flows of currency in and out of a country. A high demand for a particular currency usually means that the value of that currency will increase. Demand for a currency is created by tourism, international trade, mergers and acquisitions, speculation, and the perception of safety in terms of geo-political risk. If, for example, a company in Japan sells products to a company in the United States and the U.S.-based company would have to convert dollars into Japanese yen to pay for the goods, the flow of dollars into yen would indicate a demand for Japanese yen. If the total of currency flow led to a net demand for the Japanese yen, then the yen would increase in value.
Currencies are traded around the clock - 24 hours per day. Even though morning in Tokyo occurs during U.S. nighttime, trade and banking continue around the world. Therefore, as banks around the world buy and sell currencies, the value of currencies remain in fluctuation. Interest rate adjustments in different countries have the biggest effect on the value of currencies because investors typically look for safe investments with the highest yields. If an investor can earn 8.5% interest on deposits in England, but can pay 1% interest for the use of money in Japan, then the investor would pay to borrow the Japanese yen in order to buy the British pound. Such trades take place all the time and in very large numbers.
(For more on this topic, see Get To Know The Central Banks and Forces Behind Exchange Rates.)