"The North American Industry Classification System (NAICS) is the standard used by Federal statistical agencies in classifying business establishments for the purpose of collecting, analyzing, and publishing statistical data related to the U.S. business economy."
The NAICS is available online.
The Standard Industrial Classification (SIC) system classifies businesses by type of product or service. While this system has been replaced with the NAICS as of 1997, some directories still have SIC indexes.
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