Credit
Press this for any credit card — Visa, Mastercard, Amex, Discover. The card cost is added to the customer's total.
Debit
Press this when the customer is using a debit card, even if it runs "as credit" on the reader. Debit can never be surcharged. Your cost on those sales comes out of your side.
Cash
No card cost at all. If you're on cash-discount pricing, the customer pays the lower shelf price.
If you're not sure
Ask: "Is that a debit or a credit card?" Most people know. If they don't and the card is declined on Credit, ring it again on Debit.