A debit card allows you to buy things with money from your account, while a credit card allows you to buy things with money borrowed from a credit card company that you must repay in the future.
Credit, including credit cards, is money lent, usually for a fee, that MUST be repaid at a future time. You can borrow money or use credit to purchase something that you want now and pay for it with future income. A debit card, on the other hand, is usually connected to your bank account. When you use your debit card, you are using your own money, rather than borrowing from a credit card company and having to repay it in the future.