It's the market. It's that simple.
More precisely, FOREX is a currency trading market, and it's one of the largest and most rapidly developing markets on the planet. Over 2.5 trillion dollars are turned over on Forex every single day. That's more than 100 times more than the amount turned over daily on NASDAQ. If you're intrigued, you can click here and get more detailed market information from E-Global Trade & Finance Group, Inc.
So, what's a market? Simple: it's a place where goods are traded. Forex is no different, but with one little twist: the goods traded on Forex are the national currencies of the world's countries. For example, on Forex you might pay in American dollars and buy some Canadian dollars. Or, you could sell your Euros for Japanese Yen. There's nothing more to it than that.