Everything You Wanted To Know About Google PageRank:

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Google's PageRank Explained and how to make the most of it
by Phil Craven

PageRank is a numeric value that represents how important a page is on the web. Google figures that when one page links to another page, it is effectively casting a vote for the other page. The more votes that are cast for a page, the more important the page must be.
What is PageRank?

To calculate the PageRank for a page, all of its inbound links are taken into account. These are links from within the site and links from outside the site. ...
How is PageRank calculated?

Poor internal linkages can cause a site to fall short of its maximum but no kind of internal link structure can cause a site to exceed it. The only way to increase the maximum is to add more inbound links and/or increase the number of pages in the site.
Internal linking

Dangling links are simply links that point to any page with no outgoing links. They affect the model because it is not clear where their weight should be distributed, and there are a large number of them.
Dangling links

Inbound links (links into the site from the outside) are one way to increase a site's total PageRank. The other is to add more pages. Where the links come from doesn't matter. Google recognizes that a webmaster has no control over other sites linking into a site, and so sites are not penalized because of where the links come from.
Inbound links

Outbound links are a drain on a site's total PageRank. They leak PageRank. To counter the drain, try to ensure that the links are reciprocated.
Outbound links

The toolbar value is a good indicator of a page's PageRank but it only indicates that a page is in a certain range of the overall scale.
Toolbar PageRank

It is better to standardize the url you use for the site's home page. Otherwise each url can end up with a different PageRank, whereas all of it should have gone to just one url.
Tips

It used to be thought that Google gave a Pagerank boost to sites that are listed in the Yahoo! and ODP (a.k.a. DMOZ) directories, but these days general opinion is that they don't. There is certainly a PageRank gain for sites that are listed in those directories, but the reason for it is now thought to be this:- ...
Miscellaneous

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Everything You Wanted To Know About Google PageRank:
Google's PageRank Explained and how to make the most of it
by Phil Craven

see also:
Google Pagerank Explained, Defined, Best Reference Resources
by Robin Good