CVV stands for Creditcard Validation (or Verification) Value.
The CVV is a 3 or 4 digit code embossed or imprinted on the reverse side of many credit or debit cards. This is an extra security measure to ensure that you have physical possession of the credit card itself.
VISA, MasterCard, Discover: The CVV code is the last 3 digits after the credit card number on the back of the card in the signature area. If your card does not have a CVV code or you can not read the CVV code, please contact your credit card issuer.
American Express: The CVV code is the 4 digits after the credit card number on the back of the card in the signature area.