The market determines the value, also known as an exchange rate, of the majority of currencies. Foreign exchange can be as simple as changing one currency for another at a local bank. It can also involve trading currency on the foreign exchange market. For example, a trader is betting a central bank will ease or tighten monetary policy and that one currency will strengthen versus the other.

When trading Forex, you'll see that both 'Bid' and 'Ask' prices are quoted. The bid price is the price at which you can buy the currency, while the ask price is the price at which you can sell it. If you are purchasing a currency in a trade, this is known as a long trade, and the hope is that the currency pair will increase in value, so that you can sell it at a higher price and make a profit on the difference.

Forex, or the foreign exchange market (also called FX for short) is the marketplace where currencies are traded. At its simplest, a foreign exchange transaction might be, for example, when you transfer your local currency to a new one for an upcoming holiday. Across the market as a whole, an estimated 5.3 billion USD is traded every day between governments, banks, corporations, and speculators.
One unique aspect of this international market is that there is no central marketplace for foreign exchange. Rather, currency trading is conducted electronically over-the-counter (OTC), which means that all transactions occur via computer networks between traders around the world, rather than on one centralized exchange. The market is open 24 hours a day, five and a half days a week, and currencies are traded worldwide in the major financial centers of London, New York, Tokyo, Zurich, Frankfurt, Hong Kong, Singapore, Paris and Sydney—across almost every time zone. This means that when the trading day in the U.S. ends, the forex market begins anew in Tokyo and Hong Kong. As such, the forex market can be extremely active any time of the day, with price quotes changing constantly.