Definition of NAICS code;
The North American Industry Classification System (NAICS) is the standard used by the Federal government to classify business establishments for the purpose of government contracting and statistical analysis.

Definition of SIC code;
The Standard Industrial Classification (abbreviated SIC) is a United States government system for classifying industries by a four-digit code. ...

Classification of a business according to industry as defined by the US Department of Commerce, using a four-digit code,. For some purposes, fewer than eight digits may be used. ...