CVV2 (Card Verification Value), CVC2 (Card Validation Code) and CID (Card Identification #) codes are a new authentication scheme established by credit card companies to further efforts towards reducing fraud for internet transactions. It consists of requiring a card holder to enter the CVV2, CVC2 or CID codes in at transaction time to verify that the card is on hand. You can find these codes as shown in pictures below.

Visa, MasterCard, AmEx and Diners Club cards have CVV2, CVC2 and CID security codes that are 3 or 4 digits in length, and they are located on the front and back of the card. (see diagrams below).