NAICS stands for North American Industry Classification System.  NAICS codes are a type of industry code used to describe an industry. NAICS codes are made up of 6 digits. 

NAICS codes look like this:

Things to keep in mind when looking for NAICS codes:

• There may not be a NAICS code to describe every industry.  If you don't find a NAICS code that describes your industry, you can still search for information using keywords. 
• For example, the terms fitness center, gym, exercise center, and athletic club can all be used to describe a place where people go to work out.  If you search a business database for the term "fitness center," you may miss sources that use one of the other terms to describe the same concept.

• The industry in which a company operates (particularly those companies with more than one line of business) may be described by multiple NAICS codes.  Choose the code(s) that you think best match the industry, and see if the search results match what you're looking for.

Source: NAICS Tutorial

To find a NAICS code, start with the U.S. Census Bureau’s NAICS website.

• Use the search box marked 2017 NAICS and type in a keyword to find the 6 digit code. For example: coffee or fitness center.
• The result page will show you a list of codes for you to use related to what you typed into the search box.
• Write the code down so you can use it to search the library databases.