Postal Boundaries
The United States Postal Service (USPS) has established a national standard used for the delivery of mail to properties. This usually consists of two lines, the first being the street address and the second defines the post office to which the mail is directed for that address.
Street Address
The street address forms an important part of identifying a specific property. For more details see Addresses.
State
USPS recognizes the official political boundaries used to define the 50 US States. See Political Boundaries
Postal City
USPS aggregates zip codes into postal cities. These postal cities do not always match the legal name of the city in which a property falls. As a result, there is often a lot of confusion that can exist as to the name of the "town" or "city" to which a property belongs. 
Zip Code
USPS breaks the US into ~42,000 zip codes, each assigned a unique five-digit number. The first digit of the zip code represents one of nine zones starting in the North East (0) and ending in the North West (9). Each state is allocated a contiguous range of zip codes. The first three digits represent a mail facility, sectional center, used to sort and process mail. The last two digits identify individual post offices or delivery points serviced by the sectional center. Zip codes are defined to optimize the delivery of mail by USPS. As a result, zip codes can span political boundaries such as counties and states. e.g. 97635 coverspart of CA and OR.
Zip Plus 4
Zip Plus 4USPS also defines an additional four-digit Zip Plus 4 for many properties that can be combined to form a nine-digit zip code. Zip Plus 4 is used by USPS as a way to reduce errors in the delivery of mail, by narrowing down the location of a specific address that is not otherwise easily understood. Because they are used as an aid to the delivery of mail, the zip plus 4 can be used as a convenient way to group properties into smaller areas as an alternative to Census Blocks.
