A credit card is a payment card issued to users as a system of payment. It allows the cardholder to pay for goods and services based on the holder's promise to pay for them.
The card number's prefix, called the Bank Identification Number, is the sequence of digits at the beginning of the number that determine the bank to which a credit card number belongs. This is the first six digits for MasterCard and Visa cards. The next nine digits are the individual account number, and the final digit is a validity check code. (wikipedia)
The Credit Card Generators produces random generated credit card numbers considering card's prefixes for the different existing flags and the mod 10 algorithm in order to it's last digit, called check digit. All credit card numbers are fictional.
This tool was developed for programmers and testers who have the constant need to enter different credit card number in developing forms.