The foreign exchange market is the world's largest financial market, with an average daily trading volume of US$6.6 trillion. Forex trading involves the buying of a currency and the selling of another at the same time. This means FX trading always involves currency pairs. For example, if you have US dollars (USD) and you need Japanese Yen (JPY), you have to sell the US dollars to be able to buy the Japanese Yen.
The rate of exchange between different currencies are constantly fluctuating mainly due to the supply and demand between buyers and sellers.
The forex market may also be affected by high-impact events such as central bank decisions and by the release of economic data from different countries.