What borrowing really costs over time.
Compound interest is the same force that grows savings — pointed at you. Each period, interest is charged on the new, larger balance, so the cost accelerates the longer a debt lives. That is why time in debt is so expensive.
Play with the frequency: monthly compounding costs a little more than annual at the same quoted rate. Small differences in rate and time become big differences in dollars — which is exactly why comparing APRs before you borrow matters so much.
APR is the yearly rate before compounding; APY includes compounding, so it is slightly higher when interest compounds more than once a year. Lenders quote APR — compare APRs.
More frequent compounding means interest starts earning (or costing) interest sooner. Monthly compounding costs a bit more than annual at the same nominal rate.
Almost all consumer loans compound. Simple interest (charged only on the original principal) is rare — if you see it, that loan is cheaper than it looks.
As APR — but watch for origination fees and points, which are costs the APR may not fully capture. Compare total cost, not just the rate.