A Card Verification Code, or CVC, is a number that provides extra security to credit and debit card holders, in case an unauthorized person gets a hold of your account number. CVCs are one way to make sure someone has the actual card in his or her possession.
The location of the CVC and number of digits varies depending on card type.
American Express: the CVC is the four digits located on the front of the card, on the right side.
Discover, MasterCard, Visa, and Visa Electron: the CVC is the last three digits of the number that appears on the back of your card in the signature bar.