Credit card interest is the cost of borrowing money when you use a credit card to make purchases. When you don't pay off your balance in full by the due date, the credit card company charges interest on the remaining amount. This interest is usually expressed as an annual percentage rate (APR), which can vary based on your creditworthiness and the card issuer's policies. The interest is calculated on a daily basis, meaning that the longer you carry a balance, the more interest you will accrue. To avoid paying interest, it's best to pay off your balance each month. Understanding credit card interest can help you manage your finances more effectively.