There are two classification systems in use:
1. North American Industrial Classification System – NAICS
2. Standard Industrial Classification – SIC (supposedly replaced by NAICS)
These systems were developed to facilitate the collection and analysis of data.
Companies engaged in the same activity, whatever their size or type of ownership, are assigned the same code. Often companies have more than 1 code, but based on their revenues there should be 1 primary code indicated.
Each system is a series of hierarchical number codes that group companies by the types of products or services they make available.
More information can be found at the NAICS website.
Industry information is often organized by these codes.