1 Answer

  The United States Postal Service established ZIP (Zone Improvement Program) Codes in 1963 to help better distribute the growing amount of mail in the United States. The five-digit ZIP Codes begin with a digit from 0-9 that represents a region of the U.S. "0" represents the northeastern U.S. and "9" is used for the western states. Each subsequent digit of the ZIP code further refines the geographic area to help pinpoint the correct post office.Using ZIP Code-based geographic data is not an excellent choice since the USPS indiscriminantly assigns ZIP Code boundaries, which are subject to change at any time, and do not represent true communities or neighborhoods. Unfortunately, ZIP Code data is not appropriate for many geographic purposes.

http://geography.about.com/library/faq/blqzzip.htm