When you pay with your debit card the money is directly withdrawn from your bank account (to which the card is connected).
When you pay with a credit card you postpone the payment; you actually borrow money from the bank/creditor. Then of course you have to pay the bank/creditor back. It will be more expensive since there will be an interest rate on the borrowed money. Some cards have very high interest rates
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Konsumenternas Bank- och finansbyrå