SIC: Developed in 1987 by the US Government, the Standard Industrial Classification (SIC) system is made up of four-digit codes that identify the industry sector(s) in which a business operates.

The first two numbers identify a broad industrial sector, such as manufacturing or transportation. The last two numbers correspond to a line of business within that broad sector.

NAICS: The NAICS codes (an acronym for the North American Industry Classification System) is a classification system of six-digit codes developed jointly by Canada, Mexico and the US to identify the industry sector(s) in which a business operates.

NAICS codes were created in the 1990s with a focus on growing service and technology economy industries. The system was designed to replace the older, outdated US Standard Industrial Classification (SIC) industry code system.

Hoover's Industry: Hoover's Industry classification system (known as Hoover's Industry Codes) is a list of industries developed by the Hoover's industry experts to compliment and update the various classification systems that have been developed by governmental bodies (SICs and NAICs, for instance).

Hoover's Industry Codes are generally more comprehensive than government codes in emerging industries such as technology and software.