Mailing lists are a way to reach selected groups of people - by specific parameters (or markets) - through the Post office. The population may be broken into sections organized buy their unique qualities such as "catalog purchasers" or "business owners," or by geographic area such as businesses or residents in a particular city or zip code, or buy select statistical parameters such as businesses with over 20 employees, or heads of household with income of over $80,000. Direct Mail Lists are then sold to direct marketing companies wishing to make an offer to this specific segment of the population. By tightly specifying the parameters of the mailing list, direct mail is a very effective way to market to a precisely selected group with little wasted advertising expense.