Denominated in foreign currencies is not centered on the stock exchange, in other words, the New York Stock Exchange, and this happens all over the world via telecommunications. Trade is open 24 hours a day from Sunday afternoon to Friday afternoon (00:00 GMT Monday to Friday 22:00 GMT.
Almost every time zone around the world, are the dealers who quote all major currencies. After deciding what currency the investor wants to buy, it does so through one of these dealers (some of which can be found online). This is a fairly common practice for investors to speculate on the price of money by getting a credit card (which are available with a minimum capital of $ 500), and greatly increase their earnings and potential losses. This is called marginal trading.