Foreign exchange, or forex, is the conversion of one country's currency into another. In a free economy, a country's currency is valued according to the laws of supply and demand. In other words, a currency's value can be pegged to another country's currency, such as the U.S. dollar, or even to a basket of currencies. A country's currency value may also be set by the country's government.
However, most countries float their currencies freely against those of other countries, which keeps them in constant fluctuation.
The value of any particular currency is determined by market forces based on trade, investment, tourism and geo-political risk. Every time a tourist visits a country, for example, they must pay for goods and services using the currency of the host country. Therefore, a tourist must exchange the currency of his or her home country for the local currency. Currency exchange of this kind is one of the demand factors for a particular currency.
Another important factor of demand occurs when a foreign company seeks to do business with another in a specific country. Usually, the foreign company will have to pay in the local company's currency. At other times, it may be desirable for an investor from one country to invest in another, and that investment would have to be made in the local currency as well. All of these requirements produce a need for foreign exchange and contribute to the vast size of foreign exchange markets.
Foreign exchange is handled globally between banks and all transactions fall under the auspice of the Bank for International Settlements (BIS).
For further reading on this topic, see our Forex Tutorial.