Digital currency trading is merely the exchange of digital currencies. You can buy and sell a digital currency for another, like Bitcoin for ZAR. If a buyer’s buy order matches that of a seller’s sell order, a trade will happen automatically. 

For example, there is a buy order for a quantity of 0.2 BTC at 150 ZAR, and if you place a buy order for 0.1 BTC at 150 ZAR, you will be performing a trade. The quantity of the buy order will now decrease to 0.1 BTC at 150 ZAR.