Many industry sources include categories of companies -- typically, by an industry code. In the US, in 1997, the North American Industry Classification System (NAICS) was adopted. 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. Before 1997, the Standard Industrial Classification (SIC) was used; SIC coding strengths were for manufacturing industries.
Online sources have expanded/enhanced these numbers to provide for more granularity. You can search for companies by these codes. Note, though - sources might have different codes for companies, and codes don't work as well for conglomerates. The commercial sources have added specific key words to search.
NAICS - North American Industry Classification System.
Codes are 6-digits. Census Bureau website has bridge tables from this industry coding system (NAICS) to former code (SIC).
Concordances - NAICS to other codes - SIC, etc.
NAPCS - North American Product Classification System - also from the Census Bureau
Standard Industrial Classification Search
SIC codes are 4-digits. Keyword searching for the 1987 edition. This site is from OSHA (Dept. of Labor).
