Some spam mail includes 'ADV:' in the title. This indicates that it is part of the system used in the US to allow spam mail but to highlight that it is an advertisement. You can then make an informed choice as to whether to read or delete the message.
ADV: also allows users of email systems that have filtering facilities, such as Outlook, Eudora or Pegasus, to set a rule that will automatically remove the message. The way this works is that some email systems allow you to define a set of rules that are applied to incoming email messages. For example, a message might be automatically routed to a separate folder (perhaps a folder called 'junk') or automatically deleted. Dedicated software is also available to monitor email and remove unwanted messages.