All web shops are obliged to ask for your CVC (Card Verification Code) when you make a credit card payment. This is an additional security check and intended to prevent credit card fraud. The Card Verification Code is a unique number that is linked to your credit card number.
The location of this CVC is different for each credit card:
MasterCard, Visa: the last 3 black digits of the sequence of digits on the right-hand side in the signature panel on the back of your card.
American Express: security code on the front of the card, either to the left or right of the embossed 15-digit card number. These four digits are printed in black, not embossed.