APR
The yearly interest rate a lender charges, expressed as a percentage.
APR — short for Annual Percentage Rate — is the price tag on borrowed money. Lenders are legally required to disclose it (in the US, the Truth in Lending Act of 1968 forced this; in the UK it's the Consumer Credit Act).
The word "annual" matters. APR is what the loan would cost over a year if the rate never changed and you paid no fees. That makes it useful for comparing two products, but it doesn't tell you what you'll actually pay in month one — which is what most people experience.
For most consumer debt, APR is broken into a daily rate (APR ÷ 365) and applied to your balance each day. So a 24% APR card with a $1,000 balance for 30 days costs you about $1,000 × (24% / 365) × 30 = $19.73 in that month alone.
Example
A credit card with 24% APR charges roughly 2% interest per month on whatever you haven't paid off yet.
Why this matters
APR is the headline number that determines how much credit costs you. A 24% APR card and an 8% personal loan are not in the same universe.
The catch
APR is the *yearly* rate, but credit cards apply roughly 1/12th of it every month — and they charge interest on the new total each month, not just the original balance. That's why a $5,000 balance at 24% APR can balloon if you only pay the minimum.