The first 2 digits of the NAICS represent general categories of economic activity, or sectors. The third digit designates the subsector, fourth digit stands for the industry group the fifth digit designates the NAICS industry and the sixth digit represents the national industry.
The first 2 digits of SIC stand for the major group (there are 11 total major groups), 3 digits represent the industry group and 4 digits designate the industry code.
On the NAICS website there are 3 different boxes for searching depending on the year you want the industry information for, 2012, 2007, 2002.
SIC/NAIC codes are standardized to the four or six digit level. Some sources will add additional digits to drill down to more specific industry level. The additional numbers read like decimals and represent a more specialized industry. These additional digits may or may not be uniform in different resources.
