The international foreign exchange market (also referred to as Forex, FX, or currency market) is a global decentralized market for the trading of international currencies.
Market participants are seeking to exchange a foreign currency for their own, like multinational corporations which must pay wages and other expenses in different nations than they sell products in. However, a large part of the market is made up of currency traders, who speculate on movements in exchange rates, much like others would speculate on movements of stock prices. Currency traders try to take advantage of even small fluctuations in exchange rates.
Forex exchange is quite similar to the stock trading market, where a stock trader will buy a stock if they think its price will rise in the future and sell a stock if they think its price will fall in the future. Similarly, a forex market trader will buy a currency pair if they expect its exchange rate will rise in the future and sell a currency pair if they expect its exchange rate will fall in the future.
For example, imagine a situation where the U.S. dollar is expected to weaken in value relative to the euro. A forex trader in this situation will sell dollars and buy euros. If the euro strengthens, the purchasing power to buy dollars has now increased. The trader can now buy back more dollars than they had to begin with, making a profit.