In every foreign exchange transaction, you are simultaneously buying one currency and selling another. In effect, you are using the proceeds from the currency you sold to purchase the currency you are buying. Furthermore, every currency in the world comes attached with an interest rate set by the central bank of that currency's country. You are obligated to pay the interest on the currency that you have sold, but you also have the privilege of earning interest on the currency that you have bought. For example, let’s look at the New Zealand dollar/Japanese yen pair (NZD/JPY). Let’s assume that New Zealand has an interest rate of 8% and that Japan has an interest rate of 0.5% In the currency market, interest rates are calculated in basis points. A basis point is simply 1/100th of 1%. So, New Zealand rates are 800 basis points and Japanese rates are 50 basis points. If you decide to go long NZD/JPY you will earn 8% in annualized interest, but have to pay 0.5% for a net return of 7.5%, or 750 basis points. 

Currencies are traded on the Foreign Exchange market, also known as Forex. This is a decentralized market that spans the globe and is considered the largest by trading volume and the most liquid worldwide. Exchange rates fluctuate continuously due to the ever changing market forces of supply and demand. Forex traders buy a currency pair if they think the exchange rate will rise and sell it if they think the opposite will happen. The Forex market remains open around the world for 24 hours a day with the exception of weekends.