Although I am still appalled you've never heard of credit cards.

They're pieces of plastic, by the way. People usually whip them out when they want to buy high priced items they don't actually have the cash for. The merchant swipes the card, they sign a slip acknowledging they're going to pay for it in the future (usually in monthly installments). Unfortunately, a lot of people never do. They just let the bills keep coming, and the interest adds up. And the bill collectors never get their money.