The Card Security Code (CSC), sometimes called Card Verification Value or Code (CVV or CVC), is a security feature for credit or debit card transactions, giving increased protection against credit card fraud.

Examples include:

Generally on a VISA or Mastercard, this is the last three digits of the number near your signature line on the reverse side of your card.

References for further information:
http://en.wikipedia.org/wiki/Card_Security_Code