See also Currency Pairs.
A currency is a unit of exchange that can be used to purchase goods and/or services in one or more countries. With the notable exception of the European Union, most countries have a monopoly over the production and supply of their respective currencies.
Depending on the way a country manages its currency in the foreign exchange market, it can be either fixed rate (as in China) or floating rate (as in the United States) with respect to other currencies. In a fixed rate regime, the currency's value is matched to the value of another currency, a basket of currencies or some other measure of value (like gold) - in such cases, the country's central bank will trade the currency to maintain the fixed exchange rate - for example, by agreeing to always exchange a fixed amount of the country's currency for a fixed amount of the currency to which it's pegged. In a floating rate regime, supply and demand dictates the currency's exchange rate, and the currency's value will fluctuate on the basis of the country's balance of trade, deficits, interest rates, and other factors
Current exchange rates are called spot prices. In contrast, Exchange rates set for currency transactions in the future are called forward prices. These prices are generally specified in forward contracts that are used by cross-border companies to hedge against currency risk.