NAICS codes replaced Standard Industrial Classification (SIC) codes for most purposes in 1997.
NAICS codes are published -- with definitions and a table comparing NAICS and SIC codes -- in the 1250-page North American Industry Classification System--United States, 1997 (a/k/a the "NAICS Manual"). The codes and comparison table are posted on the Web -- without definitions -- by the U.S. Census Bureau at www.census.gov/epcd/www/naics.html.
You can search for NAICS (or SIC) codes, order the NAICS Manual and related publications, get more information about the codes and/or see an Implementation Schedule on the NAICS Web site, posted on the Web at www.naics.com. You can search through the full text of the NAICS manual on Lexis (BUSREF/NAICS).