Currency trading is supposed to be the largest market on the planet. It is estimated that in excess of $2 trillion US Dollars (USD) is traded every day. Compare this to the New York Stock Exchange's daily transactions of approximately $50 billion USD, and one can see that the magnitude of the currency trading market exceeds all other equity markets in the world combined.
The practice of currency trading is also commonly referred to as foreign exchange, Forex or FX for short. All currency has a value relative to other currencies on the planet. Currency trading uses the purchase and sale of large quantities of currency to leverage the shifts in relative value into profit.
There are two reasons the relative value of a currency fluctuates. The first is because of a "real" market: as outside investors or visitors wish to buy things within a country, they are forced to convert their domestic currency into the currency of the country in which they are buying. Similarly, as money leaves the country, people must sell their currency for the foreign currency they will need to spend or invest abroad.
The second force for currency fluctuation is speculation. When investors think a given currency will act strongly or weakly, they will buy or sell accordingly. This speculation can have drastic consequences on a national currency, and consequently on a country's economy. During the East Asia Crisis in 1997, for example, as nations in Asia began facing economic downturns, speculators used currency trading to realize enormous profits and, in many analysts' view, helped to exacerbate the problem.
Currency trading has many very real benefits over equity trading like the stock exchange. The spreads for currency trading are extremely low, making the cost to a trader very low as well. The volatility of the currency market is extremely high, which means that a trader can generate enormous return on a given exchange. The ratio of volatility to spread is approximately 500:1 for the currency trading market, as compared to 100:1 for even the most ideal of stocks.
Until recently, the currency trading market was very closed to small investors. Banking conglomerates and large multinationals were the main movers of this market place. In the past few years, however, new technologies have opened the doors to investors of all stripes. It is difficult to miss the enormous benefit of this "new" market for the individual investor: higher returns with lower risk given the same amount of market knowledge have a very small downside.