The Card Security Code (CSC) is a 3 or 4 digit number on the back of your credit card. It is used as an extra verification that you are in fact the official credit card holder. This to reduce fraud, especially on the internet. Usually you will enter the CSC number when ordering online to verify that you are in posession of the physical card.
The card appears on most (but not all) major credit and debit cards. It is shown as a 3 digit or 4 digit code, separate from the credit card number itself. The bank can ensure the card account is legitimate, because the CSC appears only on the card and not on receipts or statements. Also, it is not allowed for online shops to store this code, so it should only be used during the transaction.
Each credit card company has its own name for the CSC code, but it functions in the same manner for all major card types. (VISA refers to the code as CVV2, MasterCard calls it CVC2, American Express, and Discover call it CID.)
The back panel of most cards show the number at the end. Different names apply for the code though:
Some banks, show the last four digits of the account number followed by the 3-digit code.
Not all U.S. issuing banks, international issuing banks and/or processors currently support this security system, so it may or may not be available for a given transaction.