The foreign exchange market - Forex, FX, or currency market- is a global decentralized or Over-The-Counter -OTC- market for the trading of currencies.
The FX market determines the foreign exchange rate - buying, selling and exchanging currencies at current or determined prices.
https://www.thebalance.com/what-is-currency-trading-1344894
http://www.forextips.com/forex-101/what-is-currency-trading/