The ZIP Code is the primary postal division of the United States, established by the United States Postal Service (USPS) for the purpose of efficiently delivering the mail. Each ZIP Code represents the geographic area-of-delivery of a single Post Office. For this reason, ZIP Codes are assigned without regard for the make-up and size of the populations they serve. ZIP Codes, in general, are relatively large units (they are generally larger than census tracts). For example, the average population of a ZIP-level postal zone is six or seven thousand people; although single ZIP Codes with as many as a hundred thousand residents are not unusual. Also, these zones are relatively unstable, as they are frequently altered by the USPS to accommodate population changes, the convenience of mail carriers, and any considerations that will make mail delivery more efficient and effective.
The considerations in creating postal zones are, naturally, often contrary to the creation of census areas, and therefore, ZIP Code boundaries generally don't correspond to the census geographic boundaries, and in fact, often overlap counties, census tracts, and sometimes even states.