One of the many factors that search engines look at when determining the relevance and importance of your site amongst the others that pertain to a search parameter is link analysis: how many other sites link to yours. As mentioned else where in this SEO guide, site popularity equates for many search engines as site importance.
The first and best way to encourage people to link to your site is to have up-to-date, interesting content. Every time a website or blog or Facebook post points to your page, it adds to your site's "popularity." And search engines take notice.