An industry is classification that refers to a group of companies that are related in terms of their primary business activities. In modern economies, there are dozens of different industry classifications, which are typically grouped into larger categories called sectors.
Individual companies are generally classified into industries based on their largest sources of revenue. For example, an automobile manufacturer might have a small financing division that contributes 10% to overall revenues, but the company will still be universally classified as an auto maker for attribution purposes.
The United States government classifies industries and assigns each a code number in order to promote the comparability of statistics. NAICS (North American Industry Classification System) codes were adopted in 1997 to replace the older Standard Industrial Classification (SIC) system. The codes can be searched using online databases and web resources to find specific industry information.
Industry codes have been revised over time and some code numbers have changed. It is important to be aware when comparing data from various publishers over a length of time. Starting with the 1997 Economic Census, the Census Bureau began using a new North American Industrial Classification System (NAICS) (pronounced "nakes") which is similar to international codes. Many commercial publications have swithced to NAICS, but there are some that still use SIC codes.
Search for industry codes here: https://www.census.gov/naics/