The U.S. Census Bureau publishes the official NAICS Manual, and offers a searchable version on its website. You can also download the entire manual in PDF format.
The NAICS Association, a trade organization, provides a database of NAICS codes that can be searched for free.
The North American Industry Classification System (NAICS) is a list that groups similar businesses together and assigns a number to each category. The U.S. government uses these classification codes to organize data collected about various industries.
Some Library databases can be searched by NAICS code. Here are a few examples of how you may see those categories and their numbers displayed.
The broad category "Manufacturing" is assigned the numbers between 31 and 33, so the types of businesses listed in this category each begin with those digits. Then more numerals are added to narrow down the category. Let's use companies that make backpacks and their components as our example.
Here is how it looks in the IBISWorld Industry Research database:
