Industry codes are a way to help classify industries into categories to group similar firms and companies together. Using an industry code to search will help ensure that you are actually looking at companies that are in similar lines of business.
There are several industry code systems, but the one that is most likely to be useful to you is the North American Industry Classification System, or NAICS.
When searching using an industry code, the more digits you use, the more specific your search will be. We recommend starting your search with only 4-5 of the numbers.