Forex — short for “foreign exchange” — is the worldwide marketplace for trading currencies. Currencies are always traded in pairs, because buying one currency means selling another at the same time. It is the largest and most actively traded financial market in the world, operating 24 hours a day, five days a week. Prices in forex move based on factors like interest rates, economic data, and global events. For traders, forex is significant because of its high liquidity and constant activity, though those same qualities also mean prices can move quickly.