What is the Card Security Code?
The card security code is a unique three or four-digit number printed on your debit/credit card. This number is not embossed on the card and hence not printed on receipts - making it difficult for anyone other than the genuine cardholder to know it. Some card issuers refer to this number as the 'Card Security Code' others as the 'personal security code' and others as 'Card Verification Value'. 

Visa / Mastercard / Discover
have a three-digit number printed at the top of the signature strip on the 
reverse of the card.

American Express 
have a four-digit number printed on the front of the card, above the account number.
If your card has too many digits printed on the card, you need the last three or four since the others are probably the last digits of your card number.