When you place an order with a credit card there is a verification code, or "v-code" for short, which is associated with your credit card. This is specific to your card.
By including the verification code with your credit card details you are verifying that it is your card. This protects both the customer and the merchant. Because it is not embossed on the card in the same way that your number is, the verification number will not appear on any printed receipts. This makes it very difficult for anyone other than the cardholder to know the verification code.
The verification code is found on different places on different types of credit card. If you own a VISA or a MasterCard, you will find the three-digit verification code printed on the back of the card, at the top of the signature strip where you signed your card. If you own an American Express card, you will find the four-digit verification code printed on the front of the card, just above the embossed credit card number.