The most common form of direct marketing is direct mail, sometimes called junk mail, used by advertisers who send paper mail to all postal customers in an area or to all customers on a list.

Typical junkmail.
Any low-budget medium that can be used to deliver a communication to a customer can be employed in direct marketing. Probably the most commonly used medium for direct marketing is mail, in which marketing communications are sent to customers using the postal service. The term direct mail is used in the direct marketing industry to refer to communication deliveries by the Post Office, which may also be referred to as "junk mail" or "admail" and may involve bulk mail.
Junk mail includes advertising circulars, catalogs, free trial CDs, pre-approved credit card applications, and other unsolicited merchandising invitations delivered by mail or to homes and businesses, or delivered to consumers' mailboxes by delivery services other than the Post Office. Bulk mailings are a particularly popular method of promotion for businesses operating in the financial services, home computer, and travel and tourism industries.
In many developed countries, direct mail represents such a significant amount of the total volume of mail that special rate classes have been established. In the United States and United Kingdom, for example, there are bulk mail rates that enable marketers to send mail at rates that are substantially lower than regular first-class rates. In order to qualify for these rates, marketers must format and sort the mail in particular ways - which reduces the handling (and therefore costs) required by the postal service.
Advertisers often refine direct mail practices into targeted mailing, in which mail is sent out following database analysis to select recipients considered most likely to respond positively. For example a person who has demonstrated an interest in golf may receive direct mail for golf related products or perhaps for goods and services that are appropriate for golfers. This use of database analysis is a type of database marketing. The United States Postal Service calls this form of mail "advertising mail"