PR or PageRank (Page Rank) is the «puzomerki» Google search system. And if you want to gain the approval of the search engine, it is necessary to ensure that the PR of your site has been as high as possible, out of a maximum 10. So what is PageRank?
PageRank Technology: PageRank estimates the importance of web pages, based on an equation that includes more than 500 million variables and two billion terms. Instead of counting direct links, PageRank considers a link from page A to page B as a vote for Page B by Page A. Then, the number of votes obtained PageRank determines the significance of this page.
PageRank evaluates the importance of each page, taking part in the vote. Upon receiving votes from pages with higher relevance link it becomes more valuable. Relevant pages rank higher PR and appear at the top of search results. Google uses a total intelligent web tools to determine the significance of the page. The human factor or manipulation of results is not possible, and so that users trust Google.
Source: the Google — Technology Review