Many business publications classify their listings according to Standard Industrial Classification (SIC) codes, but these are quickly being replaced by North American Industry Classification System (NAICS) codes. Both will make research faster and more efficient. Check out these Web sites for more information on both.
*http://www.theodora.com/sic_index.html : look up SIC codes
*http://www.census.gov/epcd/naics/naicscod.txt : look up NAICS codes
*http://www.census.gov/epcd/www/naicstab.htm : convert SIC to NAICS codes