Credit Card Security Codes
All credit and debit cards carry a security code number. This number known to the bank and printed on the card, but is not stored or printed anywhere else. So, it can be used as a check that the person using the card to make a purchase is in physical possession of the card, or has at least seen the card at some time.
To find the number, look at the signature strip on the back of your card. The security code is the three digit number at the top-right corner of the strip.