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.
NAICS was developed under the auspices of the Office of Management and Budget (OMB), and adopted in 1997 to replace the Standard Industrial Classification (SIC) system.
Here's a helpful site from the Census Bureau- North American Industry Classification System: FAQ. The page answers frequently asked questions about NAICS codes, provides a NAICS code look-up search feature, and an email contact for further questions and help with identifying codes.
There are several new Industry databases on this list. Give them a try!