Industry codes, also known as Standard Industrial Classification (SIC) codes, are four digit numerical representations of industries and businesses. These codes allow for the categorization and organization of businesses for list building purposes. By using industry codes, businesses can accurately target their desired audience for marketing and sales efforts. It is important to note that SIC codes have been replaced by North American Industry Classification System (NAICS) codes in recent years. However, many databases still use SIC codes, so it is helpful to be familiar with both systems.