Article marketing is an online promotion method that involves writing articles related to your business and publishing them on other websites (directories, blogs etc.), with a link back to your site.
Here is the Wikipedia definition:
Article marketing is a type of advertising in which companies write and distribute short articles to a range of outlets such as article banks, forums, and newsletter publishers. Its main purpose is to gain a huge number of online audiences and boost the number of sales opportunities for products or services on websites.
It is one of the most successful types of marketing a business can do. This old type of marketing strategy can help marketers acquire new visitors and increase sales on their websites. Article marketing is also one of the least costly ways to market a company, the Wikipedia entry advises.