Activities and responsibilities
Functions of a central bank may include:
implementing monetary policies.
determining Interest rates
controlling the nation's entire money supply
the Government's banker and the bankers' bank ("lender of last resort")
managing the country's foreign exchange and gold reserves and the Government's stock register
regulating and supervising the banking industry
setting the official interest rate – used to manage both inflation and the country's exchange rate – and ensuring that this rate takes effect via a variety of policy mechanisms
Monetary policy
Central banks implement a country's chosen monetary policy. At the most basic level, this involves establishing what form of currency the country may have, whether a fiat currency,
gold-backed currency (disallowed for countries with membership of the International Monetary Fund),
<As the world reserve currency, the Federal Reserve is basically in charge of the IMF currency board or a currency union. When a country has its own national currency, this involves the issue of some form of standardized currency,
which is essentially a form of promissory note: a promise to exchange the note for "money" under certain circumstances.
Historically, this was often a promise to exchange the money for precious metals in some fixed amount. Now, when many currencies are fiat money, the "promise to pay" consists of the
promise to accept that currency to pay for taxes.
A central bank may use another country's currency either directly (in a currency union), or indirectly (a currency board). In the latter case, exemplified by Bulgaria, Hong Kong and Latvia, the local currency is backed at a fixed rate by the central bank's holdings of a foreign currency.
The expression "monetary policy" may also refer more narrowly to the interest-rate targets and other active measures undertaken by the monetary authority.
http://en.wikipedia.org/wiki/Central_bank