What is Google Page Rank 
Each Language has a set number of words we use therefore there are millions of pages on the web that essentially have the same physical words on them. Â Â By using keyword density, Tags, Titles and other on page factors, Google can narrow this down to just a mere couple hundred thousand relevant pages.
In order to further refine the results and provide the most credible source of information Google assigns a point of credibility ranked on an exponential scale from 0-10 where 10 is the best.  It is more difficult to go from a 6 to a 7 than it is to go from a 2 to a 3. The page with the highest relativity in combination with the highest credibility (Page Rank) will typically gain the highest positioning in the results.
Google uses your backlink structure to determine your Page Rank.  The quantity and quality of your backlinks ultimately determines your score. Each link counts as a vote towards increasing the quantity or quality of your backlink structure. In most cases, the more links you have the better.
It is important to not gain a significant amount of links from “bad neighborhoods†such as link farms because Google’s spiders are smart enough to recognize attempts to alter their ranking system and can penalize your site.
The Page Rank that is visible through the Google Toolbar is only updated about 4 times a year while the real Page Rank is a number that is continuously fluctuating depending on the links that come and go. This can help explain why your link building efforts don’t seem to have any impact on your visible Page Rank.
Other search engines such as Yahoo and MSN use a similar technique for judging a site’s credibility. Because of this system, link building is a crucial part to any search engine optimization strategy.
Thanks,
Brad Henry