Google uses a method known as PageRank to quantify the “importance” of a web page. PageRank is measured on a scale of 1 to 10 and is primarily determined by the PageRank of links pointing to a given page. An exhaustive but technical explanation can be found here. PageRank is but one of several factors Google uses to place a web page in the search engine results pages.
Google uses an algorithm called PageRank to rank web pages that match a given search string. The PageRank algorithm computes a recursive figure of merit for web pages, based on the weighted sum of the PageRanks of the pages linking to them. The PageRank (PR) thus derives from human-generated links, and correlates well with human concepts of importance.
Search engines 
