It is a new feature for credit cards designed especially for Internet security. It now appears on the back of most Visa and MasterCards, and on the front of American Express cards. The security code helps validate that the account is legitimate and the customer placing the online order actually has the credit card in their possession.
The security code is only printed the card and it is not contained in the magnetic stripe information nor does it appear on sales receipts or billing statements - a huge safeguard for the card holder. In theory, these numbers are only visible to you. When you enter your Card Security Code, you assist the merchant in verifying that the order is being placed by you.
If this code is printed on your card, it must be entered. Visa, MasterCard, and American Express now require Internet stores to obtain the security code for all cards that have a code printed on them. In order for this transaction to be accepted and processed, you must enter this code if it's printed on your card. Remember, this is done for your protection.
Flip your card over and look at the signature box. You should see either the entire 16-digit credit card number or just the last four digits followed by a special 3-digit code. This 3-digit code is your Card Security Code.
Look for the 4-digit code printed on the front of your card just above and to the right of your main credit card number. This 4-digit code is your Card Security Code.