What is a CVC?
A Card Verification Code, or CVC (also called a CVV, CV2, CSC, or others) is an extra code printed on your credit card. It is usually the last three-digit number on the signature strip on the back. On American Express (AMEX) cards, it is usually a four-digit code on the front.
The CVC is not embossed, unlike the card number and expiration date, or printed on receipts, and is an extra measure to protect against credit card fraud.
This page has pictures to help locate the code on most common card types.