The Card Verification Number/Code anti-fraud system was created so that if someone gets possession of your credit card numbers they also need to have possession of your physical card in order to use the card. American Express calls it the CID (Card Identification Number). For MasterCard, it is the CVC2 (Card Validation Code). Visa calls this number the CVV2 (Card Verification Value).
The verification number is a 3-digit number printed on the back of your card. It appears after and to the right of your card number.
The American Express verification number is a 4-digit number printed on the front of your card. It appears after and to the right of your card number.