Google pagerank is a measure of the number and importance of pages that are linking to the page (with the public page rank lowered by Google if they don't like some things the site/page does). One measure obviously has huge limitations for capture the nature or value of any page (for more info see previous post). But it does tell you something. But I still find it fun to look at the pagerank values - except when they go down for my sites :-( Now I also track the similar Moz Page Authority and can take some solace if the MozPA goes up :-)
I didn't pay much attention at first to the SEO Moz ranking details. I tried to figure out what they were measuring then gave up because it wasn't clear and figured MozRank must be the equivalent of pagerank. Well MozRank isn't (I still can't figure out what it is a measure of - maybe primarily just measuring links without factoring in the "authority" the links) but anyway Moz Page Authority is the measure that is equivalent to Google PageRank. So in the chart below the MozRank is shown inside [ ] for October 2011, while Jan 2013 I updated to MozPA /10 (because SEO Moz also decided to scale MozPA up to 100 while Google PageRank caps out at ten).
* internal pages
** new url, old url forwarded
- didn't exist yet
[blank] I don't know what the pagerank was, sometimes the site didn't exist yet.
***I have noticed my high pagerank blog posts can be at PR of 4 for years and then have no page rank at all. Meanwhile MozPA doesn't seem to disappear the authority of those page. The PageRank can often reappear in another update (I even think they may come back in between updates but I might be mistaken).
Related: Google Page Rank Updates for November 2012 - Google and Links - Using Twitter Data to Improve Search Results