The credit card security verification code (CSC) is usually a 3 or 4 digit number, which is not part of the credit card number. The CSC is typically printed on the back of a credit card (usually in the signature field).
Validation of this number is done during transactions to check for fraud. On some cards, all or part of the card number appears before the CSC, for example, 1234 567. In this example, 1234 are the last four digits of the credit card number, and 567 is the CSC.
The number appears on the card itself (not on receipts or statements). Hence, it provides some assurance that the physical card is in the possession of the buyer.