APR (annual percentage rate)

The interest rate tells you what the lender charges on the balance. The APR folds in certain upfront costs, such as origination fees, discount points, and (on mortgages) mortgage insurance, and spreads them over the loan's term. That's why a mortgage's APR is usually a bit higher than its interest rate.

The Truth in Lending Act requires lenders to disclose the APR, so it's the best single number for comparing offers with different fee structures. It isn't perfect: it assumes you keep the loan for its full term, so a loan with high fees looks better on APR than it really is if you refinance or sell early.

On credit cards, the APR is simply the interest rate, since cards don't have upfront loan fees.