The daily traded volume in the forex market is in excess of $5 trillion. Unlike the commodities and other markets, forex does not have its own exchange and is an over the counter product. For many traders, because of its size, it is thought to be the purest market out there.
Forex is always traded in pairs, meaning that you would choose to either buy or sell the value of one country's currency in exchange for another, for example EUR/USD. In this currency pair for instance, if you believe that the value of EUR will rise against USD, you would go long or buy EUR/USD. Conversely, if you believed that EUR will fall against USD, you would go short or sell EUR/USD. When trading forex, the first currency is always known as the base currency while the second currency is called the counter currency.