Forex as a word derives from Foreign Exchange and, as the name signifies, is the marketplace where national currencies are traded against each other. Forex is, by far, the biggest market in the world, with a trading volume of $5 trillion on a daily basis. Trading Forex requires the opening of a short/sell or long/buy trade of a pair of currencies (i.e. EUR/USD - Euro against US Dollar).
Short/sell is the trade that predicts that the price of the currency pair is going to fall while the long/buy is the speculation that it is going to rise. The prices of the currency pairs are affected by news of the markets, politics, etc. You can trade Forex along with 500+ other instruments with your Live or Demo account with NAGA.