Google used to push major visible PageRank updates about quarterly and do very little, if any, pagerank changes at other times. Pagerank is a value given to the links coming into a web page on a logarithmic scale. So a PR of 2 is 10 times greater than 1 and 100 less than PR 4. It is named for Google co-founder and current CEO Larry Page. Google doesn't do the quarterly updates anymore (they have been less frequent) and seems to make many more adjustments to PageRanks all the time.
Now the visible PageRank is just a way of Google letting other people see the relative PageRanks of web pages. The real pagerank Google updates much more frequently. The real pagerank is what is used as a factor in search results. I also believe Google now includes penalties to visible pageranks that are not actually used in their search rankings. So they can show a visible indication that the site is not as highly ranked as it may be (this is most easily noticeable when pages have a reduction in pagerank with no real world explanation).
PageRank matter but is hardly a huge factor. It is much more important to have links from related pages than just high pagerank pages, for being ranked highly in Google search results. However a link from a page with a PR of 7 would be much better than and a very similar page with a PR of 1. Pagerank "passed" to the linked to page is divided by the total number of links on the page. So if you had a link from a relavent PR 7 page with 10 links that would provide 5 times more "pagerank" than the same PR 7 with 50 links.
Another thing I have noticed much more over the last few years is having PR for internal pages is much harder. Many sites with PR of 3 or even 4 on the homepage may have no other pages with PR. It seems to me, that this is true far more than the math would indicate, so my guess is Google is reducing PR internal often in some way I have no idea about. Again remember the displayed PR google provides doesn't have to correspond to the real one they calculate and use.
The displayed pagerank is more fun than a measure that can be relied on as an important measure. But I still find it fun to look at the pagerank values - except when they go down for my sites :-(
Check the current pagerank on your sites using our related site: Multiple Site PageRank checker.
* internal pages
** new url as of January 2011
[blank] I don't know what the pagerank was, sometimes the site didn't exist yet.
In my experience it is often more telling how the pagerank of internal pages. The sites that have high ranking internal pages are much rarer than site with say a pagerank of 4 or even 5 on the home page.
Also note that a PR of 4 could be nearly 10 times higher than another PR 4. For example if 3.5 is shown as 4 (Google only shows an integer) and 4.4 is shown as 4 given the logarithmic scale used the 4.4 would be 9 times greater than the 3.5. PageRanks are logarithmic and then compared to the other pages on the web. So if a site was very close to 3.5 it might show up as 3 one time then 4 then 3 again... all without any significant change. On the other hand going from the absolute lowest 3 to the absolute highest 4 could be nearly a 100 times increase.
Related: Last Google Toolbar PageRank Update of 2008 - Web Page Authority - 6 years Later Goolge Acts To Let Me Block Sites I don't want to see - PageRank Distribution - Google's Search Results - Should Factors Other Than User Value be Used