Marginal trading is simply the term used for trading with borrowed capital. It is appealing because of the fact that in FOREX investments can be made without a real money supply. This allows investors to invest much more money with fewer money transfer costs, and open bigger positions with a much smaller amount of actual capital. Thus, one can conduct relatively large transactions, very quickly and cheaply, with a small amount of initial capital. Marginal trading in an exchange market is quantified in lots. The term "lot" refers to approximately $100,000, an amount which can be obtained by putting up as little as 0.5% or $500.
EXAMPLE: You believe that signals in the market are indicating that the British Pound will go up against the US Dollar. You open 1 lot for buying the Pound with a 1% margin at the price of 1.49889 and wait for the exchange rate to climb. At some point in the future, your predictions come true and you decide to sell. You close the position at 1.5050 and earn 61 pips or about $405. Thus, on an initial capital investment of $1,000, you have made over 40% in profits. (Just as an example of how exchange rates change in the course of a day, an average daily change of the Euro (in Dollars) is about 70 to 100 pips.)
When you decide to close a position, the deposit sum that you originally made is returned to you and a calculation of your profits or losses is done. This profit or loss is then credited to your account.