The Forex Currency Pairs

Foreign Exchange trading is in general the trading of many currencies of the world. It is emerging as the largest and least regulated market providing the greatest liquidity to investors.
This trading is always done in pairs – Currency Pairs, one currency is bought and the other is sold. Together, they make up what is known as the "exchange rate".
For example, you may buy Euros with Dollars, anticipating that the Euro to increase in value relative to the Dollar. If the Euro rises relative to the Dollar, you sell the position and can earn a profit.

Most commonly traded currencies or the “majors” are:

US Dollar (USD)
Japanese Yen (JPY)
Euro (EUR)
British Pound (GBP)
Canadian Dollar (CAD)
Australian Dollar (AUD)
Swiss Franc (CHF)

Most commonly traded currency pairs are:

US Dollar and the Japanese Yen (USD/JPY)
Euro and US Dollar (EUR/USD)
US Dollar and Swiss franc (USD/CHF)
British Pound and US Dollar (GBP/USD)

While quoting currency pairs, the first currency is referred to as the base currency and the second as the counter or quote currency. The base currency is always equal to 1 monetary unit of exchange, for example, 1 Dollar, 1 Pound, 1 Euro.