Page Rank or PageRank™ is Google's proprietary formula for measuring a web page's relevance and importance. PageRank relies on what Google calls “the uniquely democratic nature of the web”. They do this by looking at outbound and inbound links on the web to get an indication of an individual page's importance or value.
 
How does it work?
 
Google interprets a link from one web site to another web site as a vote of importance. An outbound link on your web site is telling the Google spider: “The site I’m sending you to is important.” Google also analyzes the referring page (the page with the outbound link). Outbound links from web pages that are themselves important or valuable receive more weight and importance.
 
When important, well ranking pages have links to other web sites, this benefits the sites receiving the links and helps make those pages rank higher in Google search results.
 
To ensure their search results are relevant to your query, Google combines its PageRank formula with its proprietary text-matching algorithms to find pages that are both important and relevant to your search. It will look at internal link structures, HTML tags, keywords and textual relationships in content pages.