The Verification Code is imprinted on credit cards to help merchants verify transactions when the actual card is not present, such as Internet purchases or donations. The merchant uses this number as part of the authorization process with the card issuer.
Visa, Master Card, and Discover: The verification code for Visa, Master Card, and Discover is a 3-digit number printed on the back of your card. It appears after the account number or last 4-digits of your account number, in the signature panel.
American Express: The American Express verification code is a 4-digit number printed on the front of your card. It appears above and to the right of the embossed account number.