In the last years, the foreign exchange market has expanded from one where banks would execute transactions between themselves to one in which many other kinds of financial institutions like brokers and market-makers participate including non-financial corporations, investment firms, pension funds and hedge funds.

Its' focus has broadened from servicing importers and exporters to handling the vast amounts of overseas investment and other capital flows that currently take place. Lately foreign exchange day trading has become increasingly popular and various firms offer trading facilities to the small investor.

Foreign exchange is an 'over the counter' (OTC) market, that means that there is no central exchange and clearing house where orders are matched. Geographic trading 'centers' exist around the world however and are: (in order of importance) London, New York, Tokyo, Singapore, Frankfurt, Geneva & Zurich, Paris and Hong Kong. Essentially foreign exchange deals are made between participants on the basis of trust and reputation to deliver on an agreement. In the case of banks trading with one another, they do so solely on that basis. In the retail market, customers demand a written legally accepted contract between themselves and their broker in exchange of a deposit of funds on which basis the customer may trade.

Some market participants may be involved in the 'goods' market, conducting international transactions for the purchase or sale of merchandise. Some may be engaged in 'direct investment' in plant and equipment, or may be in the 'money market,' trading short-term debt instruments internationally. The various investors, hedgers, and speculators may be focused on any time period, from a few minutes to several years. But, whether official or private, and whether their motive be investing, hedging, speculating, arbitraging, paying for imports, or seeking to influence the rate, they are all part of the aggregate demand for and supply of the currencies involved, and they all play a role in determining the exchange rate at that moment.

Recommended further reading:
Advantages
Origins
Main Forex Markets
Market Dynamics
Speculation/Investing
Margin Trading
Trading with a Strategy
Risks inherent to Forex
Forex abbreviations