What is a Verification Code (CVV or CVC2)
All Visa and Master card  cards have a 3-digit verification code printed on the back of the card. This code is used to verify that the card is present at the time of purchase.

Finding the verification code on your Visa or Master card.

Finding the verification code on your American Express card.
For all American Express Cards the Verification Code is a four-digit number on the front of the card above the credit card number on either the right or the left side of your American Express credit card.