What is the CVV number on your credit card?

This is an authenticating code established by the credit card companies to help ward off fraudulent use on the internet.

When purchasing something on the internet the interface on the checkout will often ask for this code. It is located on the back of the card and usually a 3 digit code. This provides a 

cryptographic check of the embossed information on the front of the card. The back of the cards do contain the full 16-digit account number. the CVV/CVC code comes after the long number separated by a space.

Asking for the code helps, but doesn't guarantee, that the person making the purchase has the credit card in hand. VISA and Mastercard call it the CVV2. American Express calls it the CID.

You could write in "Ask for ID" next to your signature when you are checking out of an offline store. It is fun to see if the clerk asks or not.

CVV stands for CARD VERIFICATION VALUE CODE