Each currency pair can be thought of a single unit consisting of a “base currency” (the first currency) and a “counter (or quoted) currency” (the second currency) which can be bought or sold. It shows how much of the counter currency is needed to buy one unit of the base currency. So, in the EUR/USD currency pair EUR is the base currency and USD is the counter currency. If you expect the price of Euro to increase against the price of the U.S. dollar you can buy the EUR/USD currency pair. While buying a currency pair (going long) the base currency (EUR) is being bought, whereas the counter currency (USD) is being sold. Thus, you buy the EUR/USD currency pair at a lower price to later sell it at a higher price and as a result make a profit. If you expect the opposite situation, you can sell the currency pair (go short), meaning sell Euro and buy the U.S. dollar.
National central banks play an important role in the foreign exchange markets. They try to control the money supply, inflation, and/or interest rates and often have official or unofficial target rates for their currencies. They can use their often substantial foreign exchange reserves to stabilize the market. Nevertheless, the effectiveness of central bank "stabilizing speculation" is doubtful because central banks do not go bankrupt if they make large losses as other traders would. There is also no convincing evidence that they actually make a profit from trading.
Historically, different international monetary systems have emphasized different policy mixes. For instance, the Bretton Woods system emphasized the first two at the expense of free capital movement. The collapse of the system destroyed the stability and predictability of the currency markets. The resultant large fluctuations meant a rise in exchange rate risk (as well as in profit opportunities). Governments now face numerous challenges that are often captured under the term globalization or capital mobility: the move to floating exchange rates, the political liberalization of capital controls, and technological and financial innovation.
The need to exchange currencies is the primary reason why the forex market is the largest, most liquid financial market in the world. It dwarfs other markets in size, even the stock market, with an average traded value of around U.S. $2,000 billion per day. (The total volume changes all the time, but as of August 2012, the Bank for International Settlements (BIS) reported that the forex market traded in excess of U.S. $4.9 trillion per day.)