A large portion of the information available on U.S. industries and businesses is organized by two major industry classification systems: the North American Industry Classification System (NAICS) and the Standard Industry Classification System (SIC). Both are hierarchical, numerical classification systems. Internationally-focused sources often apply other industry classification systems.
Companies involved in a wide range of activities may have multiple industry classifications, although usually one industry is considered primary.
The classification systems themselves do not provide direct links to industry information but many online databases and print directories enable searching by these codes. These systems also provide useful descriptions of industries and sub-industries as well as inspiration for search terms.
Company research often also involves doing research on the industries in which the companies you are interested in are associated.