What is the CV Code?

CV coding is a new authentication procedure established by credit card
companies to further efforts towards reducing fraud for internet
transactions. It consists of requiring a card holder to enter the
CV number at transaction time to verify that the card is on hand.
The CV code is a security feature for "card not present" transactions
(e.g., Internet transactions), and now appears on most (but not all)
major credit and debit cards. This new feature is a three- or four-digit
code which provides a cryptographic check of the information embossed
on the card. Therefore, the CVV code is not part of the card number
itself.

The back panel of most Visa/MasterCard cards contain the full 16-digit
account number, followed by the CVV/CVC code. Some banks, though, only
show the last four digits of the account number followed by the code.
To aid in the prevention of fraudulent credit card use, we now require
the 3 or 4 digit code on the back of your credit card. When you submit
your credit card information your data is protected by Secure Socket Layer
(SSL) technology certified by a digital certificate.

Finding your CV Code
  Visa, MasterCard and Discover CV Code.
  American Express CV Code.