Currently, the major engines typically interpret importance as popularity – the more popular a site, page or document, the more valuable the information contained therein must be. This assumption has proven fairly successful in practice, as the engines have continued to increase users’ satisfaction by using metrics that interpret popularity.
Popularity and relevance aren’t determined manually. Instead, the engines craft careful, mathematical equations – algorithms – to sort the wheat from the chaff and to then rank the wheat in order of tastiness (or however it is that farmers determine wheat’s value).
These algorithms are often comprised of hundreds of components. In the search marketing field, we often refer to them as “ranking factors” SEOmoz crafted a resource specifically on this subject –Search Engine Ranking Factors.