The Wikipedia page about ZIP codes details the history, structure, allocation, and various uses of the ZIP code system. Introduced by the United States Postal Service (USPS) in 1963, the ZIP (Zone Improvement Plan) code system was established to ensure mail travels more efficiently and quickly. The page also discusses the 1983 extended version, ZIP+4, which provides more specific location information.
RT @SimpleGeoAPI: The history of the zip code.