APR (Annual Percentage Rate) expresses the yearly cost of a loan as a single percentage. On many products — notably mortgages — APR includes not just the interest rate but also certain mandatory fees, which is why the APR is usually slightly higher than the advertised rate.
APR is designed for comparison shopping: two loans with the same interest rate but different fees will show different APRs, revealing the truly cheaper loan. For credit cards, the APR is typically the interest rate itself, applied monthly to your balance.
See how APR drives real costs in the Credit Card Payoff Calculator.