The string of numbers on you credit card houses a batch of information. For instance, the first number tells you what kind of card it is—4 for Visa, 5 for MasterCard, and so on. Visa cards are always 16 digits long and have other identifying characteristics:

When looking at the balance of the numbers, the 2nd through 6th digits are the bank number, and the 7th-15th numbers are your account number. The remaining digit is known as the "check digit," which is used to help determine whether or not the overall number is legitimate.

[Five Cent Nickel]