FOREX - the foreign exchange (currency or forex or FX) market is the largest and the most liquid financial market with the daily volume of more than $1.5 trillion . Trading this market involves buying and selling world currencies taking the profit from exchange rate differences. Forex trading can yield high profits, but it is also very risky. Everyone can participate in Forex trading via Forex brokers.
To find out more about FOREX you can read various FOREX articles or some free FOREX e-books.
You will find a lot of information there. If you'll still have questions you could browse some FOREX FAQ or consult the FOREX Glossary.