Short for Standard Industry Classification, SIC code is a numerical way of identifying what product and services business provide for their customers. The SIC system began in 1937, when the federal government - specifically the Department of Commerce - determined it to be a smart strategy that streamlined the classification of business activities in a more uniform manner. It also helped foster communication and identification for companies in foreign countries dealing with businesses based in the U.S. While SIC coding is still in use, they've largely been replaced by a six-digit identification method, implemented by the North American Industry Classification System (NAICS).
The classification process itself is quite complex, but when business owners know what number their industry starts with, it's a bit easier. For example, as noted by SICCode.com, the first two digits of an SIC code are for the establishment's general industry group, the third represents the subsector of the overall industry group and the fourth is the number that contains the most specificity as to what the business does in terms of product or services.