Direct Marketing and Sales Support
Direct marketing is defined as a message sent to a specific person that is intended to elicit a specific reaction at a specific time. Much of what is considered to be marketing and promotion in general is really just DM.
DM Applications
Some DM is directed at existing customers, with a goal of retaining their business. Generally, one combs your customer database for individuals who meet certain criteria (have made $X in purchases in the past six months, but nothing in the past six weeks) and sends a message to them to elicit a reorder.
Other DM uses lists acquired from other sources, attempting to reach out to prospective first-time customers. Alternately, it may be directed to those who provided their e-mail address in response to a promotion, but ultimately did not make a purchase.
Online vs. Traditional DM
One of the limitations of DM in the traditional channels (primarily, mail) is that DM online is fast and traceable. For example, you can set up separate Web pages for separate campaigns to see which is most effective, or you can even set up each promotion to pass a code unique to the customer to gauge their response, and you can do detailed and accurate analysis.
The kind of copy written for direct marketing - succinct and to-the-point - tends to work very well on the Web. However, longer DM pieces do not do well on the Web - people will tolerate (and even pay attention to) longer passages of text on paper than they will online.
Online DM can make use of the multimedia capabilities of the Web, which can be more attention-getting and more effective that words and static pictures.
There is no limit to the amount of text you can put on a Web site for people who want more detail, but handle it carefully, as it will often be a barrier to those who do not.
Also, a Web site is not a "push" instrument: you must send something to the user (generally an e-mail, possible a banner ad) that gets them to come to the Web site in the first place. Think of this as a teaser, similar to the text printed on the outside of an envelope to get people to open it and read the contents.
It is much easier to deliver a message tailored to the audience on the Web, as there are no printing costs to prepare separate "versions" of a pitch for different audiences, and mailing lists are more easily segmented.
However, since e-mail is virtually free of cost, it has been terribly abused, to the point where it has largely lost its effectiveness. Most mass-mailing lists are practically useless. A better approach is to use targeted or opt-in lists, in which people have indicated in advance their interest in a topic (or item) and a willingness to receive promotional messages.
Handle sign-ups for mailing lists carefully (such that a person cannot accidentally sign up for the list), and provide a way to delist themselves if they later change their minds. A message sent to an unwilling or uninterested recipient does not generate a sale, and may even harm your chances of getting their business in the future.
Another suggestion is to "piggyback" on other mailing lists - which is the equivalent of buying advertising space in a newsletter.
Additional Online Strategies
Certain advertising networks (Double-click, at the time) post ads to an array of sites, but associate ad-views with users instead of the sites, making it possible for you to buy ads that address a specific target market rather than whatever audience a site happens to draw.
Some of the larger search engines also profile their regular users, enabling you to target a market instead of associating your advertising with search results.