Marketing any product or service online is similar to offline marketing in some respects but different in others. The goal is the same, to make a profit. A good education of marketing is necessary for both words; online I would say you need an education in direct marketing to be successful.
The benefit of direct marketing online is that you don't need a retailer involved; you just market your products or services directly to your end user. Direct marketing is how most advertising online is done, most of the major players such as Yahoo and Google run direct marketing systems.
When you do a search online with Google or Yahoo, the ads you see on the right hand side of the page are direct marketing ads. They might not be called that but the idea is to have a "searcher" click on the ad, visit the site and perform an action such as buying something or putting in their email address.
Another effective and my favorite way to market product and services online is by using article marketing. You write short articles in your field of expertise and submit them to article directories; these directories will spread them all across the Internet. Your article will have a link to your website so that a reader can visit your website after they finish reading the articles.
Article marketing is free and only cost you your time to write the article. It is effective and you are sure your prospects are interested because they read your article to get to your website.