This explains PageRank
http://en.wikipedia.org/wiki/PageRank
Basically it's a measure of a page's link popularity based on the quality and quantity of the links that point to it.
There are two types, there's the real PR which is always in a stae of flux and only viewable by Google, and there's ToolBar PR which is what is displayed on the Google Toolbar as a little green line.
The TBPR is a snapshot of what the PR was at some uspecified time (Hence why this forum has PR0, it's too new to have been ranked before the update) and is updated every 3 months or so. PR is one of many factors that can influence a page's postion in Google's search results.
Some people needlesly obsess over TBPR at the detriment of other factors. If they forget about it and do everthing else right, the PR will eventually follow.
What is diaplayed on the ToolBar is largely irrelevent unless you're in the business of selling links basd on it. If you're caught doing this, Google will punish you severly.
The only real use for TBPR IMO is that it can give the impression that your site has authority to n00bs, which if you're running an ecomerce site can be the differnce beteen you getting the sale or the other site with slightly less PR. Apart from that, it doesn't mean a lot. e.g. I have one site with homepage PR4 that gets on average 200 unique visitors a day from Google, and anoter site that was PR0 that could get 11,000+ unique Google visitors a day Doesn't take a genious to work out that the PR0 homepage site is more valuable than the PR4 one in this case.
PageRank Update
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