The Standard Industrial Classification (SIC) and North American Industry Classification System (NAICS) are numerical classification systems created by the United States government. These systems are used to group/classify like companies into industries for the purposes of collecting, analyzing, and publishing statistical data related to the U.S. business economy. The SIC system was originally developed in the 1930s and was last updated in 1987. The NAICS system was first released in 2002, updated in 2007 and last updated in 2012.
While NAICS has "officially" replaced SIC, many business directories and databases still use one or both of the systems. Therefore, it is important to know about both SIC and NAICS.
Some library databases, books, and web sites allow you to search using SIC and/or NAICS codes. It is very helpful to know what your industry code is before starting your research in Library and government resources.