A measure of the quantity and quality of sites that link to your site.
Link popularity is an example of the move by search engines towards off-the-page-criteria to determine quality content. Off-the-page-criteria adds the aspect of impartiality to search engine rankings, as citations from other authors in the Web community helps define a site’s reputation. In theory, great sites will naturally attract many links, and content-poor sites will have difficulty attracting any links.
Link popularity assumes that not all inbound links are equal. For example, an inbound link from a major directory carries more weight than an inbound link from an obscure personal home page.
see also: SEO Tools