To help guard against fraud, the major credit card companies have implemented a system to ensure the credit card used in a transaction is actually posessed by the user. The verification number is a non-embossed number printed on the credit card.
For Visa, MasterCard, and Discover cards, the number is located on the back of your card near your signature. The number on these cards is three digits long.
For American Express cards, the number is located on the front of your card, on the right hand side above your card number. The number on American Express cards is four digits long.