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.
When trading in the forex market, you're buying or selling the currency of a particular country, relative to another currency. But there's no physical exchange of money from one party to another. That's what happens at a foreign exchange kiosk—think of a tourist visiting Times Square in New York City from Japan. He may be converting his physical yen to actual U.S. dollar cash (and may be charged a commission fee to do so) so he can spend his money while he's traveling. But in the world of electronic markets, traders are usually taking a position in a specific currency, with the hope that there will be some upward movement and strength in the currency they're buying (or weakness if they're selling) so they can make a profit.
Unlike stock markets, which can trace their roots back centuries, the forex market as we understand it today is a truly new market. Of course, in its most basic sense—that of people converting one currency to another for financial advantage—forex has been around since nations began minting currencies. But the modern forex markets are a modern invention. After the accord at Bretton Woods in 1971, more major currencies were allowed to float freely against one another. The values of individual currencies vary, which has given rise to the need for foreign exchange services and trading.