What are SIC and NAICS codes?
They stand for Standard Industrial Classification and North American Industrial Classification System and they are one method of finding related businesses.
Use them in databases for finding specific kinds of businesses; i.e. show me all the restaurants or metal fabricators or insurance companies, etc. You don't have to know the codes - most databases will let you search for the right code.
One criticism is that these codes don't reflect the words we mortals use for different businesses and sometimes they also seemed mired in the industrial past.
If you're having trouble finding the right SIC/NAICS code, try to find one business and then open the database record to see what code is associated with that.
Recommended Databases for this: ORBIS, MINT