Foreign exchange market

The foreign exchange (currency or forex or FX) market exists wherever one currency is traded for another. It is by far the largest market in the world, in terms of cash value traded, and includes trading between large banks, central banks, currency speculators, multinational corporations, governments, and other financial markets and institutions. The trade happening in the forex markets across the globe currently exceeds US$1.9 trillion/day (on average). Retail traders (individuals) are currently a very small part of this market and may only participate indirectly through brokers or banks and may be targets of forex scams.

Trading characteristics

There is no single unified foreign exchange market. Due to the over-the-counter (OTC) nature of currency markets, there are rather a number of interconnected marketplaces, where different currency instruments are traded. This implies that there is no such thing as a single dollar rate - but rather a number of different rates (prices), depending on what bank or market maker is trading. In practice the rates are often very close, otherwise they could be exploited by arbitrageurs.

There is little or no 'inside information' in the foreign exchange markets. Exchange rate fluctuations are usually caused by actual monetary flows as well as by expectations of changes in monetary flows caused by changes in GDP growth, inflation, interest rates, budget and trade deficits or surpluses, large cross-border M&A deals and other macroeconomic conditions. Major news is released publicly, often on scheduled dates, so many people have access to the same news at the same time. However, the large banks have an important advantage; they can see their customers' order flow.
Currencies are traded against one another. Each pair of currencies thus constitutes an individual product and is traditionally noted XXX/YYY, where YYY is the ISO 4217 international three-letter code of the currency into which the price of one unit of XXX is expressed. For instance, EUR/USD is the price of the euro expressed in US dollars, as in 1 euro = 1.3045 dollar. Out of convention, the first currency in the pair, the base currency, was the stronger currency at the creation of the pair. The second currency, counter currency, was the weaker currency at the creation of the pair.
The factors affecting XXX will affect both XXX/YYY and XXX/ZZZ. This causes positive currency correlation between XXX/YYY and XXX/ZZZ.
On the spot market, according to the BIS study, the most heavily traded products were:

EUR/USD - 28 %
USD/JPY - 18 %
GBP/USD (also called sterling or cable) - 14 %

and the US currency was involved in 89% of transactions, followed by the euro (37%), the yen (20%) and sterling (17%). (Note that volume percentages should add up to 200% - 100% for all the sellers, and 100% for all the buyers).
Although trading in the euro has grown considerably since the currency's creation in January 1999, the foreign exchange market is thus far still largely dollar-centered. For instance, trading the euro versus a non-European currency ZZZ will usually involve two trades: EUR/USD and USD/ZZZ. The only exception to this is EUR/JPY, which is an established traded currency pair in the interbank spot market.