What is interest? How does interest affect credit card purchases?

money paid regularly at a particular rate for the use of money lent, or for delaying the repayment of a debt.

Credit cards charge interest when you don't pay off your full balance by the due date each month. When you carry, or revolve, a credit card balance from month to month, interest is charged on a daily basis, and it affects both your existing balance and any new purchases that post to your account.