This is a security feature implemented to assist with preventing fraud. The security code is not printed on any receipts and therefore helps validate that the customer making a purchase actually has physical possession of the credit card.
Visa, Master Card, & Discover: Look on the back of your credit card in the signature box. The CVV is the 3-digit code printed on the right-hand side of your 16-digit credit card number.
American Express: The CVV is the 4-digit number printed on the front of your credit card in one of the following locations: