In SEO marketing the factors not contained within a Web Page but that effect its position in the SERP and due to the simplicity with which on page optimization can be manipulated search engines now place more weight on so called off page factors. Off page optimization criteria are obtained from sources other than the website. Off Page Optimization (off page SEO) is done off the pages for a website to maximize its performance in the search engines for targeted keywords related to the On Page Optimization and keywords.Probably the most important of these criteria are the quantity of inbound link using anchor text containing your keyword. These links should come from pages covering similar theme of your website preferably from well established and authorized site

Keep in mind that Google uses all inbound-links in its ranking process, not just those shown by the 'backlink' option of the Google toolbar if the page has the luxury of many inbound-links then mixing target keywords to cover different queries is a good idea. Since Yahoo! acquired Overture and dumped the Google index it has published an index called Web Rank. This is based on the number of incoming links to a site. It would seem likely that this is also a factor in their ranking process.