[AdWords Basics] PageRank

In Google's organic results, relevancy to the user's query is determined by over 200 factors, one of which is the PageRank for a given page. PageRank is the measure of the importance of a page based on the incoming links from other pages. In simple terms, each link from site A to a page on site B adds to site B's PageRank.

http://infolab.stanford.edu/~backrub/google.html

http://sungmooncho.com/2012/08/26/pagerank/