What is Forex Trading Online?

The foreign exchange market or currency market, known informally as Forex, is an over-the-counter trading instrument where one currency is traded for another. It is the most traded market in the world, with an average turnover of $3.2 trillion per day. 
(Source: Bank for International Settlements, September 2007)

Simply put, Forex is the trading of currencies against one another. Each currency pair is traditionally noted XXX/YYY - for example, USD/JPY is the US Dollar paired with the Japanese Yen. Unlike stocks and futures exchanges, the Forex market operates 24 hours a day from Sunday 17:15 to Friday 16:30 Eastern Time (US). This allows traders to react to news when it breaks, rather than having to wait until the market is open. 

Example of Foreign Currency Trading:

USD/JPY is trading at (sell/buy) 109.47/109.50 and you believe that the USD is trending downward. You may sell the pair at 109.47.