The yearly cost of borrowing, including most fees.
APR stands for annual percentage rate: the yearly cost of money you borrow, expressed as a percentage of the amount outstanding.
On a credit card, the APR is essentially the interest rate — cards rarely carry fees that get folded in. On a mortgage, the APR is higher than the headline interest rate because it also carries points, origination charges and other lender fees, which is why it is the fairer number for comparing loan offers.
Interest on a card is normally charged daily on the balance you carry, so the effective annual cost of never clearing the card runs slightly above the stated APR.
The average card APR in the United States is 20.94% right now. Carry $5,000 on that card for a year and you pay about $1,047 in interest without reducing the balance at all.
See the card apr page