This advertising mode is known as "link building," and it involves getting other web sites to link to your site. It's like one of your neighbors recommending a good plumber or handyman; it carries more weight than if a person just stumbled across your Web site.
In today's world, there is much more to good search engine listings than simply optimizing your site for keywords. In order to keep searchers happy, search engines are always developing ways to make their results more relevant. In the last couple of years, links have become increasingly more important to the engines because they see links as an endorsement of your site by other Web sites. Think about it for a minute: Would you link to a site you didn't like?