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:
Credit card fraud is on the rise on the Internet. Requiring CVV2 codes is just one more action we take to ensure that our customers' security is protected.