NAICS (rhymes with snakes), is the North American Industry Classification System. It is a 6-digit, hierarchical code that is used to classify "lines of business" for establishments for the purpose of collecting, analyzing, and publishing statistical data. NAICS codes are replacing the Standard Industrial Classification (SIC) system.
Many of the Library's company and industry databases allow you to search by NAICS or SIC code or by keyword and may allow you to browse by industry codes.
See the NAICS Main Page, which includes a browseable hierarchy/structure of codes and a keyword search.
These databases provide aggregated financial ratios for all companies in an industry. They do not provide ratios for individual companies -- see the tab on this guide for Company Directories and Financials.