If you want to know how America's zip-code system works then you should refer to Engaging Data's Zip Code Map of the United States. Enter a five number zip-code into this map and with each digit you enter the map will show the area which your digits represent.
The zip-code system was introduced by the United States Postal Service in 1963. The five digits of a zip-code number indicate the address where a letter should be delivered. The first digit in a five digit zip-code represents a group of U.S. states. For example the number 9 (shown in the map above) is used for states along the western seaboard. The first digit numbers run roughly east to west across the USA. A first digit of 0 indicates an address in the far North-East (shown in the map below).
Each subsequent digit in a five digit zip-code indicates an ever more refined and smaller geographical area. The second and third digits together represent a region within the area indicated by the first digit. For example a large city. The fourth and fifth digits represent a group of delivery addresses within that region.
The Engaging Data map visualizes this system very neatly. As you enter each digit of a five digit postcode into the map the highlighted region of that postcode becomes ever more refined. When you have entered all 5 number of a zip-code you will have narrowed down the area to an individual postal delivery route.
Carto recently published a blog post about Why You Shouldn't Use Zip-Codes for Spatial Analysis. Even if you disagree with Carto's conclusion the article includes an interesting history of the zip-code and explanation of how the zip-code system works. Carto's main argument about why zip-codes shouldn't be used for spatial analysis is that the 5 digits don't really represent geographical areas but a collection of postal routes.
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