SIC and NAICS codes are classification systems used by the U.S. government to track business and economic activity in the U.S.
Some business databases use these codes to classify businesses by industry. You can learn about an industry or use a directory to find companies by their industry, by searching by an SIC or NAICS code.
SIC codes are older, last updated in 1987, being phased out but still used in some business directories.
NAICS codes are newer codes (1997-) developed to replace SIC codes and track U.S. economic activity.
If you are starting a business you need to learn as much as you can about the industry in which you will be operating.
An industry survey will give you an overview of an industry : how the industry operates, major companies, products, the competitive landscape, barriers to entry, current conditions, trends and more.
If you are doing business or investment research, and want to learn the basics of an industry and its market size, current conditions, trends and forecasts, try the resources listed in this guide to find an industry survey.