An exchange rate is simply a score for one currency against another and represents the number of units of one currency that need to be exchanged for a single unit of another currency. The exchange rate is thus the price of one currency against another and, given the number of world currencies today, within the US alone there are legally dozens of exchange rates. Now that seems simple enough but, unfortunately, it is not quite that easy.
Quite apart from these simple exchange rates, which are sometimes referred to as 'spot' rates, there are also a whole range of 'trade weighted' or 'effective' rates which show the movement of one currency against an average of several other currencies. There are also exchange rates which are used in markets such as the forwards markets in which delivery dates are set at some point in the future, rather than at the time of the initial transaction. In other words, there is no such thing as an exchange rate, but are in fact a series of different exchange rates depending upon the nature of the transaction.
The foreign exchange market is driven largely by supply and demand and the exchange rate between any two currencies at any moment in time is influenced substantively by the interaction of the various players in the market. In a few cases currencies are still fixed, or the exchange rate is set by the monetary authorities, and when this is the case the country's central bank will normally intervene if required and either buy or sell the currency to keep its exchange rate within a narrow and defined band. In the vast majority of cases however, and certainly in the case of the US, treaties are allowed to float and central banks do not normally, and certainly not routinely, intervene to support their currency. Correspondingly, the exchange rate for a particular currency against other currencies is determined by players, large and small, who are buying and selling the currency at any particular moment in time.
The mix of participants in the market is important and will affect different treaties to varying degrees. Some buyers and sellers deal in the market purely in support of international trade and are operating in the 'goods' market buying and selling currency to pay for merchandise being traded across national borders. Other dealers are buying and selling currencies in support of 'portfolio investment' and are trading in bonds, stocks and other financial instruments across national borders. Yet another group of currency traders are operating in the 'money' market and are trading short term debt across international borders.
As if this were not complicated enough, this mix of traders whether they are paying for imports, investing, speculating, hedging, arbitraging or simply seeking to influence exchange rates are also focusing their attention of a variety of different timeframes in their trading which will range from a matter of minutes to several years.
Against this background it is no wonder than comparable exchange rates is a complex business. Doing so however is vitally important since exchange rates the behavior of all participants in the market and, in today's open market, also influence interest rates, consumer prices, economic growth, investment decision and so much else. It is for this reason that the forex market plays such a critical role in determining exchange rates.