The CVV Number ("Card Verification Value") on your credit card or debit card is a 3 digit number on VISA®, MasterCard® and Discover® branded credit and debit cards. On your American Express® branded credit or debit card it is a 4 digit numeric code.
As the CVV is not embossed (like the card number) the CVV is not printed on any receipts, hence it is not likely to be known by anyone other than the card owner.
CVV is an anti-fraud measure being introduced by credit card companies worldwide. Your customers are required to enter the CVV number each time a payment is made and the customer is not present to sign a receipt, as in online transactions.