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 or floating rate 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 a floating rate regime, supply and demand dictates the currency's exchange rate.
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.