Page Rank,shortly known as PR.It is an algorithm created by google.Value of each link is called as Page Rank.there are several factors for calculate this PR ranking.

Google describes as "PageRank reflects our view of the importance of web pages by considering more than 500 million variables and 2 billion terms. Pages that we believe are important pages receive a higher PageRank and are more likely to appear at the top of the search results".we cannot easily calculate this PR.but we can see this using some tools like google toolbar,seo toolbar,etc....

         if you site have link with high PR sites.then you can get PR from that site.but that should be Dofollow link.If your PR is high,then you can easily get ranking first in google SERP.but some of the links we should  avoid.that is buying backlinks from banned sites and unrelated sites.

you can check your PR here..http://www.prchecker.info/check_page_rank.php
get more info..http://en.wikipedia.org/wiki/PageRank