Foreign exchange market - Wikipedia
Exhaustive explanation about the nature of the market, the history, and the development.
http://en.wikipedia.org/wiki/Foreign_exchange_market

Forex Education - BabyPips.com
The beginner's guide to forex trading. Market explained easy way.
http://www.babypips.com/

ForexOnlineLearning.com
Features tutorials and video tutorials to help you get started and resources such as software, forums and trading firms that have been personally tested.
http://www.forexonlinelearning.com/

Investopedia.com: Forex Articles and Insight
Collection of articles explaining the market of foreign exchange.
http://investopedia.com/articles/forex/

STIFX - Online Forex Education
Free forex education for live trading. Economic indicators, technical study, margin trading, and glossary.
http://www.stifxonline.com/education-forex-futures-equities-cfds-trade-learning-stifxonline.html