In currency exchange, there is always one currency that is bought and one that is sold (the sold currency is usually used to purchase the bought currency). (For mobile app click HERE).
To start trading you go to Marketplace (Marketplace explained) and when you have clicked on the desired currency pair, you will be presented with two options: Quicktrade (you can also access Quick Trade directly from Dashboard and Wallet, Quick Trade explained) and Tradeview (Tradeview explained):
The currencies bought and sold are determined by two things in the order form. Firstly, whether the "buy" or "sell" button on the order form is selected, and secondly, which currency pair is selected. If you want to buy or sell the second currency in a pair (the quote currency), you have to think backwards, since the "buy" button will actually sell the quote currency in exchange for the base currency and the "sell" button will buy the quote currency using the base currency.
If the "Buy" button is selected and currency pair x/y is selected, then currency x will be bought and currency y sold.
If the "Sell" button is selected and currency pair x/y is selected, then currency x will be sold and currency y will be bought.
Hot Trade (accessible from Dashboard, Hot Trade explained): is a section where all the most traded pairs are shown:
If you hover over the currency pair, you will also see the Quicktrade (Quick Trade explained) and Tradeview (Tradeview explained) options: