In forex, currencies are quoted in pairs. Let’s take the most popular currency pair as an example, EUR/USD. The first currency (Euro in this case) is called the base currency and the second (USD) is called the quote currency. When you trade a pair you are speculating on whether the base currency (EUR) will strengthen or weaken against the quote currency (USD).
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.
Investors – Investment firms who manage large portfolios for their clients use the Fx market to facilitate transactions in foreign securities. For example, an investment manager controlling an international equity portfolio needs to use the Forex market to purchase and sell several currency pairs in order to pay for foreign securities they want to purchase.
The need to exchange currencies is the primary reason why the forex market is the largest, most liquid financial market in the world. It dwarfs other markets in size, even the stock market, with an average traded value of around U.S. $2,000 billion per day. (The total volume changes all the time, but as of August 2012, the Bank for International Settlements (BIS) reported that the forex market traded in excess of U.S. $4.9 trillion per day.)