Imagine a trader who expects interest rates to rise in the U.S. compared to Australia while the exchange rate between the two currencies (AUD/USD) is .71 (it takes $.71 USD to buy $1.00 AUD). The trader believes higher interest rates in the U.S. will increase demand for USD, and therefore the AUD/USD exchange rate will fall because it will require fewer, stronger USD to buy an AUD.
Forex is a portmanteau of foreign currency and exchange. Foreign exchange is the process of changing one currency into another currency for a variety of reasons, usually for commerce, trading, or tourism. According to a recent triennial report from the Bank for International Settlements (a global bank for national central banks), the average was more than $5.1 trillion in daily forex trading volume.