CVV2 is an authentication mechanism created by card companies to help reduce fraud with online transactions. It requires the card holder to have the card physically in hand in order to enter three or four digits found printed on the card itself.
On Visa, MasterCard and Discover credit cards, the number is located on the back of the card in the signature area.
On American Express cards, the number is located on the front of the card, just above the card number, usually on the right. Diner's Club and some older credit cards may not have the CVV2 codes.