I would need my wife to explain it since she is a credit card using expert but I believe that a bank issued VISA credit card can be used in a store as both a debit card (you swipe and enter your pin number) or as a credit card but the bank just pays the charge off like it is a debit transaction so you don't have to do anything like send in a check to pay it off but it allows the transaction to go through without entering a pin number.
When I buy on line I never enter a pin number so it is handled like a credit card. When I go to an ATM you do enter a pin number so it is a straight debit transaction. Same card, card number, security number, etc.