Debit and credit cards both serve as a means to transact business without exchanging cash directly. This "electronic cash" is efficient, convenient and relatively safe.
The main difference between a credit and a debit card (or check card) is in the way funds are handled at the bank.
A debit card requires funds available in the associated checking account and draws funds directly for each transaction. Transactions cannot exceed funds available. Essentially a debit or check card is an electronic extension to the checking account.
A credit cards credit debt. Each transactions is applied to the outstanding balance owed. Payments are made to reduce some are all of this debt each month.