Key resources

Industry Classifications (Statistics Canada)
Links to all editions of NAICS (1997 to the present), as well as to SIC, ISIC, and NACE classification systems.Includes links to concordances or correspondence tables between editions of a single classification system and between systems.

NAICS Codes and Understanding Industry Classification Systems (US Census Bureau)
A useful overview of the structure of NAICS as it is used in the US Economic Census. See also their main NAICS page.

National Classification Systems (United Nations Statistics Division)
NAICS and SIC are only common here in North America. To learn about the systems used elsewhere, see the UNSD's list of activity, product, expenditure, and occupation systems in use by governments worldwide.

About standard classification systems

Standard classifications systems such as the North American Industry Classification System (NAICS) and the Standard Industrial Classification system (SIC) are developed by government statistical agencies as a means of grouping company and industry data to make it easier to work with. 

Commercial publishers may develop their own classification system, often based on a government system, and may indicate an SIC code or an NAICS code as part of a company description in a directory or database.  

Important:

Subjective application:
Companies are not "legally" assigned a consistent industry classification code (NAICS, SIC, or other system) that would then be used in all directories or databases. One directory may assign a specific code to a company, while another might chose a different code for the same company.

Multiple codes:
Companies can have more than one code, depending on their activities.

Multiple classification systems:
In addition, many databases and directories use more than one classification system (including proprietary ones), although NAICS tends to be one of the most commonly used among SFU's databases.