Growing Popularity & Links
By crawling between web pages via links, search engines can discern whether a website is popular or unpopular. To win this popularity contest, a website must have many links from other websites pointing toward it. The process of analysing link information has been honed by search engines exceptionally well. So much so that search engines can now produce detailed assessments of a website by examining its link structure.
Finding Trustworthy Links
Search engines work under the assumption that quality websites will naturally link to other high quality websites. Untrustworthy websites are likely to have less links then their reliable counterparts, making link counting an excellent way to determine the credibility of a website.