Visa cards have a 3 digit CW2 number, and the number is located on the back of the card.

MasterCard's have a 3 digit CW2 number, and the number is located on the back of the card.

Discover cards have a 3 digit Cw2 number, and the number is located on the back of the car

American Express cards have a 4 digit Cw2 number, and the number is located on the front of the card underneath the actual credit card number.