PR stands for page rank. It was developed at Stanford University by Larry Page (hence the name Page-Rank) and Sergey Brin as part of a research project about a new kind of search engine. In case you didn't know they are the founders of Google Inc., the company that owns Blogger too.
Anyway, there are so many tools out there on the net that checks your page rank. However, I was confused when I get different rankings on using different tools. I illustrate this using my ah ok lah blog
1. PPP PageRank/ Alexa Rank Checker gives my blog a pr of 4/10. It gives it a O/10 when I use the url http://www.ok-lah.blogspot.com as compared to one which doesn't have the www.
2. Blogflux.com's Google PageRank Checker on the other hand, gives my blog a page rank of 5/10.
3. Seomoz.org's Page Strength SEO Tool also gives my blog a page strength of 5/10.
4. CheckPageRank.com gives me a page rank of 4/10.
5. Live Pagerank gives me an average PR: 4.1 | Max PR: 5 | Min PR: 4
I am totally confused now, so will the real page ranker please stand up.