It is simply an added security feature cards use. Stores ask for the CVV2 number when you are making a purchase. They are usually located on the back side of a card.
CVV is an anti-fraud security feature to help verify that you are in possession of your credit card. For Visa/Mastercard, the three-digit CVV number is printed on the signature panel on the back of the card immediately after the card's account number. For American Express, the four-digit CVV number is printed on the front of the card above the card account number. http://www.duckware.com/order/cvv.html