What is the CVV number?

It's a 3 or 4 digit verification number found only on the card itself to assure the card number is genuine. The CVV number is a safety feature that tells the merchant that a valid credit card was physically in the credit card holder's possession when they placed the order.

Where do I find the CVV number?

On American Express it's above the credit card number on the front of the card. For Visa, MasterCard, and Discover it's to the right of the credit card number, on the back of the card.