ZIP codes in the United States entered use on July 1, 1963.
"ZIP" stands for "Zoning Improvement Code". It refers to the fact that ZIP codes were an expansion on a system of postal zones that were used in larger cities starting in 1943.
Decoding a zip code: The first digit designates a broad geographical area of the United States, ranging from zero for the Northeast to nine for the far West. The next two digits specify population concentrations within those areas. The last two digits designate small post offices or postal zones in larger zoned cities.
ZIP+4 was introduced in 1983. The extra four digits further specify the location. According to the US Postal Service "The sixth and seventh numbers denote a delivery sector, which may be several blocks, a group of streets, a group of post office boxes, several office buildings, a single high-rise office building, a large apartment building, or a small geographic area. The last two numbers denote a delivery segment, which might be one floor of an office building, one side of a street between intersecting streets, specific departments in a firm, or a group of post office boxes."
Some interesting zip codes: 22222 is Arlington, Virginia; 44444 is Newton Falls, OH; 55555 is Young America, MN and 12345 is Schenectady, New York.