For those confused over the difference between the PIN option and the credit option of a debit card, this MarketWatch article
does a good job explaining the difference as part of its review over the debit-card fee debate that's taking place now in Congress. Here's an excerpt:
Merchants' pay less to the issuing bank if you use your debit card's personal identification number, or PIN, rather than signing the receipt. When the cashier says "debit or credit?" she's asking whether you want to use your PIN for debit or to sign for credit.