Some investment management firms also have more speculative specialist currency overlay operations, which manage clients' currency exposures with the aim of generating profits as well as limiting risk. While the number of this type of specialist firms is quite small, many have a large value of assets under management and can, therefore, generate large trades.
Fluctuations in exchange rates are usually caused by actual monetary flows as well as by expectations of changes in monetary flows. These are caused by changes in gross domestic product (GDP) growth, inflation (purchasing power parity theory), interest rates (interest rate parity, Domestic Fisher effect, International Fisher effect), 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, large banks have an important advantage; they can see their customers' order flow.
Just like stocks, you can trade currency based on what you think its value is (or where it's headed). But the big difference with forex is that you can trade up or down just as easily. If you think a currency will increase in value, you can buy it. If you think it will decrease, you can sell it. With a market this large, finding a buyer when you're selling and a seller when you're buying is much easier than in in other markets. Maybe you hear on the news that China is devaluing its currency to draw more foreign business into its country. If you think that trend will continue, you could make a forex trade by selling the Chinese currency against another currency, say, the US dollar. The more the Chinese currency devalues against the US dollar, the higher your profits. If the Chinese currency increases in value while you have your sell position open, then your losses increase and you want to get out of the trade.
There are actually three ways that institutions, corporations and individuals trade forex: the spot market, the forwards market, and the futures market. Forex trading in the spot market has always been the largest market because it is the "underlying" real asset that the forwards and futures markets are based on. In the past, the futures market was the most popular venue for traders because it was available to individual investors for a longer period of time. However, with the advent of electronic trading and numerous forex brokers, the spot market has witnessed a huge surge in activity and now surpasses the futures market as the preferred trading market for individual investors and speculators. When people refer to the forex market, they usually are referring to the spot market. The forwards and futures markets tend to be more popular with companies that need to hedge their foreign exchange risks out to a specific date in the future.