One of the biggest and most active sector of Forex is currency market, it has trading volume of Over 5 Trillion a Day.
Each transaction in the currency market involves two different trades: the sale of one currency and the purchase of another. The two currencies involved in the trade are known as a pair. While it is possible to swap virtually any currency for another, the majority of trading occurs among a handful of popular currency pairs.
Currency

Market Share

USD 57.6%
EUR 20.3%
JPY 11.6.3%
GBP 3.8%
AUD 3.3%
CAD 3.1%
CHF 2.8%
CNY 2%