A currency is a unit of exchange, facilitating the transfer of goods and/or services. It is one form of money, where money is anything that serves as a medium of exchange, a store of value, and a standard of value. Currencies are the dominant medium of exchange. Coins and paper money are both forms of currency.

Category: Image - :WorldCurrency.JPG|thumb|left||250px|Currencies around the world.

In most cases, each country has monopoly control over the supply and production of its own currency. (Member countries of the European Union's Economic and Monetary Union are a notable exception to this rule, as they have ceded control of monetary policy to the European Central Bank.) To facilitate trade between these currency zones, there are exchange rates, which are the prices at which currencies (and the goods and services of individual currency zones) can be exchanged against each other. Currencies can be classified as either floating currencies or fixed currencies based on their exchange rate regime.

In cases where a country does have control of its own currency, that control is exercised either by a central bank or by a Ministry of Finance. In either case, the institution that has control of monetary policy is referred to as the monetary authority. Monetary authorities have varying degrees of autonomy from the governments that create them. In the United States, the Federal Reserve System operates without direct oversight by the legislative or executive branches. It is important to note that a monetary authority is created and supported by its sponsoring government, so independence can be reduced or revoked by the legislative or executive authority that creates it. However, in practical terms, the revocation of authority is not likely. In almost all Western countries, the monetary authority is largely independent from the government.

Several countries can use the same name for their own distinct currencies (e.g., dollar in Canada and the United States). By contrast, several countries can also use the same currency (e.g., the euro), or one country can declare the currency of another country to be legal tender. For example, Panama and El Salvador have declared U.S. currency to be legal tender, and from 1791?1857, Spanish silver coins were legal tender in the United States. At various times countries have either re-stamped foreign coins, or used currency board issuing one note of currency for each note of a foreign government held, as Ecuador currently does.

Each currency typically has a main currency unit (the U.S. dollar, for example, or the euro) and a fractional currency, often valued at of the main currency: 100 cents = 1 dollar, 100 centimes = 1 franc, 100 pence = 1 pound, although units of or are also common. Some currencies do not have any smaller units at all.

Mauritania and Madagascar are the only remaining countries that do not use the decimal system; instead, the Mauritanian ouguiya is divided into 5 khoums, while the Malagasy ariary is divided into 5 iraimbilanja. In these countries, words like dollar or pound "were simply names for given weights of gold." Due to inflation khoums and iraimbilanja have in practice fallen into disuse. (See non-decimal currencies for other historic currencies with non-decimal divisions.)