The CSC (Card Security Code) is a 3 or 4 digit number that appears on your credit card, but is not part of your credit card number. The CSC is also know by a few different names depending on the credit card - Card Verification Code (CVC) by Visa, Card Validation Code (CVV) by MasterCard, Card Identification Number (CID) by American Express.
The CSC is an extra security feature of your credit card. Since the CSC is never printed as part of the receipt or on credit card statements, this ensures the card must be present in order to make a purchase.
In the signature box on the back of your Visa you should see a 16-digit credit card number followed by a special 3 digit code. This 3 digit code is your CSC.
On the front of your card next to your main credit card number look for a 4 digit code. This 4 digit number is the Card Security Code.