The card is run through a scanner and the bank electronically verifies that the funds are available before approving your transaction. An on-line debit card is similar to an ATM transaction. An off-line debit card is also known as a check card , because it is similar to the way a check is signed and paid. The card is run through a scanner, but instead of entering your PIN, you sign a receipt like a credit card sale. The funds aren't deducted from your account for a few days.
Debit cards have several advantages.
You don't have to show identification to use them. You don't have to carry cash or a checkbook. You don't pay interest charges. Merchants like them because checking out is faster and easier and their processing fees are usually less than credit card fees. Debit cards also have several disadvantages.