Take a quick reading from Wikipedia about foreign exchange trade.
Forex, FX or foreign exchange is a trade of buying and selling currencies. This trade is done by personal, corporations, banks and even governments. Forex market opens 24 hours except on weekends and it has many factors that affect the exchange rates. Forex trading is not a quick rich scheme and it requires high competency of business and political analysis from the trader (a person who trades).
Forex market has different levels of players. The participants in Forex market are banks, hedge funds (large private investors), companies (mostly multi-national corporations who need to pay employees in different countries), investment management firms, retail Forex broker, and retail traders (individual traders who participate through broker or banks).
The traded currencies mostly are United States dollar USD ($), Euro EUR (€), Japanese yen JPY (¥), British pound sterling GBP (£), Swiss franc CHF (Fr), Australian dollar AUD ($), Canadian dollar CAD ($), Swedish krona SEK (kr), Hong Kong dollar HKD ($), Norwegian krone NOK (kr).
Foreign exchange rates fluctuate based on supply and demand of certain currencies in certain countries. This supply and demand is influenced by economic and political conditions around the world. It may be inflation, economic growth, regional conflict, and many other factors.
