Interest expenses

Banks generally borrow the money they then lend to their customers. As they receive very low-interest loans from other firms, they may borrow as much as their customers require, while lending their capital to other borrowers at higher rates. If the card issuer charges 15% on money lent to users, and it costs 5% to borrow the money to lend, and the balance sits with the cardholder for a year, the issuer earns 10% on the loan. This 5% difference is the "interest expense" and the 10% is the "net interest spread".