Investopedia Says: There is no central marketplace for currency exchange, rather, trade is conducted over-the-counter. The forex market is open 24 hours a day, five days a week, with currencies being traded worldwide among the major financial centers of London, New York, Tokyo, Zürich, Frankfurt, Hong Kong, Singapore, Paris and Sydney - spanning most time zones.
The forex is the largest market in the world in terms of the total cash value traded, and any person, firm, or country may participate in this market.
The result of all this international trade is that financial institutions accumulate surpluses of different currencies from loan repayments by foreign borrowers, and also from import-export trade financing on behalf of bank customers.
The interbank foreign exchange market is an over-the-counter market, a network of commercial banks, central banks, brokers, and customers who communicate with each other by telex and telephone throughout the world's major financial centers. Foreign exchange traders also make markets or speculate in different currencies, usually anticipating future appreciation of stronger currencies against weaker ones, through the foreign exchange Forward Market and the Currency Futures market.