CVV2 and CID

"CVV" stands for "Card Verification Value". Discover Card and American Express call it the "Cardmember ID").

CVV2 and CID numbers are security measures that helps protect you. Since a CVV2 number is listed on your credit card, but is not stored anywhere, the only way to know the correct CVV2 number for your credit card is to physically have possession of the card itself. All VISA, MasterCard and American Express cards made in America in the past 5 years or so have a CVV2 or CID number.

 
For Visa, MasterCard and Discover, the 3 or 4 digit Security Code (CVV2) can be found on the back of your credit card.
 
For American Express, the Security Code (CID) can be found on the front of the card.