A Card Verification Value, or CVV, 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. CVVs are one way to make sure someone has the actual card in his or her possession. The location of the CVV and number of digits varies depending on card type. For Visa, MasterCard and Discover branded credit and debit cards, the CVV is three digit number following the account number on the back side of the card.