When Google hit the seen a few years back their revolutionary algorithms transformed search and resulted in breakthroughs in search relevancy. What was so different about Google? Many things and the algorithms are tweaked every week in this ever-evolving behemoth of public software. Still, when Google hit the seen, one significant difference separated it from the pack, more than anything else - link popularity.

Google determined that sites should not just be evaluated by on-page criteria such as title and meta tags, since webmasters had already taken to abusing these tags in the name of search engine optimization. Instead, it was determined that links from external websites should be evaluated to determine public opinion as to the relevancy of a website and this should weight Google’s on-page relevancy determination.
The idea was simple: each link to a site is like a vote, and similar to a democratic election, those votes should be tallied to determine which site is the most important. Once this was factored in, it became much more difficult for webmasters to ‘game’ Google’s algorithms. Moreover, this also gave Google a window into which websites were not only relevant, but which ones others thought were truly the best of their breed. It was this emphasis on link popularity more than anything else that gave Google a leg up and led to it becoming the King of search.
Today, nearly every major search engine denotes inbound links when determining ranking of a website. Moreover, an entire sub-culture and link-bartering culture has arisen amongst web masters to allow the search engine optimizer to once again gain a footing in controlling how well their website ranks in the search engines. Link popularity has become so important that today, a webmaster has virtually no hope of ranking well in a competitive space without partaking in these link bartering schemes.