One key difference between Forex and the stock market is that the former runs 24 hours a day. Forex trading takes place constantly, throughout the day and only stops on weekends, and only because the volume of orders is too low. So technically, the Forex market is never actually closed for business.
In finance, a trade is an exchange of a security (stocks, bonds, commodities, currencies, derivatives or any valuable financial instrument) for "cash", typically a short-dated promise to pay in the currency of the country where the 'exchange' is located. The price at which a financial instrument is traded, is determined by the supply and demand for that financial instrument.