The concept of Page Rank (PR) is devised by Google, which gives a unique ranking to every page on internet based on the number and quality of inbound links pointing at a page. Although it is a Google tool, its patent rights are reserved with Stanford University.
To define PR in terms of Google:
“Page Rank relies on the uniquely democratic nature of the web by using its vast link structure as an indicator of an individual page’s value. In essence, Google interprets a link from page A to page B as a vote, by page A, for page B. But, Google looks at more than the sheer volume of votes, or links a page receives; it also analyzes the page that casts the vote. Votes cast by pages that are themselves “important” weigh more heavily and help to make other pages “important” ”.
In other words, a hyperlink to a page acts as a vote of support. A page that is linked by a number of highly ranked pages gets high rank itself.
Let us consider an example:
In the above diagram, Page B will receive a better PR than Page M; because Page B is connected by and to a more important page i.e. Page A which is further linked by three more pages, Pages C, D and E. This shows that Page A is a more valued page than page L which is referenced just by a single page i.e. Page N.
Although this is a very simple description of far more complex procedure of interlinking billions of web pages on the internet. But this does not mean that all links are equal, a few links from important pages make a good affect on PR than the entry of many unimportant links.
Once installed, Google Toolbar a page rank score for every visited page.for example:
This means Google values Yahoo’s home page more than Rediff’s one. Again if the Page Rank bar in the Google Toolbar is completely grey, it means the page has not yet been ranked by Google. Hence it is not considered important in the eyes of Google.
Although PR contributes towards ranking of a site in the top of the search engine but a top ranked site might not necessarily contain good PR. It is considered as just one of the many pre-requisites to climb up on the ladder of search engine rank.