Forex (foreign exchange) is the world's most traded and largest financial market in the world. It is a place to trade currencies from different countries. For instance, you can buy Euro with the US Dollar or sell Japanese Yen with Pound Sterling. As the transactions involve two different currencies, forex is always traded in pairs. Some of the most traded pairs in forex are EUR/USD, GBP/USD, and USD/JPY.
Forex is traded 24 hours a day, 5 days a week. It is the biggest financial market in the world with a daily volume of $6.6 trillion per day.