A currency zone is a country or region in which a specific currency is the dominant medium of exchange. To facilitate trade between currency zones, there are exchange rates i.e. 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 common usage, currency sometimes refers to only paper money, as in "coins and currency", but this is misleading. Coins and paper money are both forms of currency.
In most cases, each country has monopoly control over its own currency. Member countries of the European Monetary Union are a notable exception to this rule, as they have ceded control of monetary policy to the European Central Bank.
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 operates with full independence from the government. 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. In almost all Western countries, the monetary authority is largely independent from the government.