Foreign exchange trading (also known as forex, or FX) is the buying and/or selling of one currency in relation to another.

Given all currencies are priced differently, the price difference can make you a profit or a loss when the values of currencies change.

The FX market is estimated to have a turnover value of over $5 trillion a day; which is quite a bit. Individuals, companies, banks, funds, and governments buy and sell global currencies all day every day until Friday - where the market closes at 22:00 GMT, and reopens on Sunday at 22:00 GMT. This $5 trillion turnover can be the result of import/export activities, hedging against currency risk, or speculating on the change of currency values.

Given you can normally exchange currencies through an exchange office or bank, forex trading is not centralised in a major national exchange, like stocks and futures. Such setup makes it very flexible, customisable, and of course, accessible.