Siva's blog

One of the amazing Google product for webmaster is it PageRank(PR). It is used to know the website unique content, unique visitors, organic visitors, link popularity, search results , SERP and many. People who are working with Search Engine Optimization, Domain(or website) buyer makes note of these details. Google PR is preferred more than Alexa because you can trick (or trigger) to get some decent Alexa Rank.

It should also be noted that PR is not unique for a website, for example, you may have PR3 in Home page while another webpage in your website may have PR5, It is because of the popularity of the article in that webpage

. Google update their rank every 3 to 4 months. The Last Google PR update was around January first week 2010. This week(April 3rd), google released the next update.
Here is a one of a video(by Matt Cutts) that Google uses to display Quality searches.

One of the best way to check website PR stat is by simply installing Google Toolbar and you can see the PR in the toolbar.
More info : http://www.google.com/corporate/tech.html

Update from Buckdat:
The upcoming PR update Schedule for 2010:
1. January 2010
2. March 2010
3. June 2010
4. October 2010
5. December 2010