A Card Verification Value, or CVV, is a number that provides extra security to credit and debit card holders, in case an unauthorised person gets a hold of your account number. CVVs are one way to make sure someone has the actual card in his or her possession.
The CVV on American Express cards is four digits, and is located on the front of the card, on the right side. Discover, MasterCard, and Visa use three-digit CVVs, which are listed on the back of the card. The CVV is the last three digits of the number that appears on your signature bar.