Currency markets (also known as forex markets) are the largest day trading markets (in terms of volume and amount of money), trading approximately $2 trillion per day. Currency markets are the markets where one currency is traded for another currency, such as trading the Euro for the US Dollar.
The majority of currency trading is between central banks, commercial banks, and large companies, with the largest currency trader currently being Deutsche Bank in Europe, but the currency markets are also traded by individual day traders. Currency markets are unique in that they are not traded at exchanges, but are traded directly between traders instead.
There are several large currency trading centers, with London in Europe, New York in the US, and Tokyo in Japan, being the largest.