Created by the U.S. Census Bureau in the 1930's, SIC codes are numbers that classify industries (i.e. business activities, like types of manufacturing and services).
SIC codes are hierarchical -- the longer the number, the more detailed the industry. Officially, SIC codes are up to four digits long. However, eight-digit SIC codes are used by some sources (ex. Dun & Bradstreet) to define more specific industries.
The Census stopped used SIC codes after 1987. NAICS has replaced the out-of-date SIC structure. However, a few databases and reference books still use SIC codes.
Examples of SIC codes:
23 Apparel made from fabrics and similar materials 232 Men's and boys' furnishings, work clothing, and allied garments 2325 Men's and boy's separate trousers and slacks
This five-minute video introduces NAICS (North American Industrial Classification System) codes and demonstrates how to identify an industry's code using the Census web site. SIC codes are covered, too.