Know the Industry Classification
Every industry has a classificaton developed by the US Office of Management and Budget.
(North American Industry Classification System)
(Standard Industrial Classification)
NAICS is an update of SIC and provides more detailed industry breakdowns. Both systems are still in use, so knowing both codes will help you to more easily access information in a variety resources.
Find the Industry Classifcation of a Company
Find Companies in a particular Industry
Match a NAICS code to an SIC code (& vice versa)
Use these Concordances from the NAICS website