APR (annual percentage rate) is the yearly cost of borrowing, including interest and certain fees, expressed as a percentage.
Lenders in the US must disclose APR so you can compare loans. For credit cards, APR is essentially the interest rate. For mortgages it includes some fees, so it is usually a bit higher than the quoted rate.
Why it matters to you
Comparing APRs, not just monthly payments, is the simplest way to see which loan is cheaper.