On the other hand, Alexa Rank, as far as I know is based only on one factor - traffic. This simply means that more the amount of visitors to your site, better will be your ranking. However, few people complain that the rankings are biased between the ones who have their toolbar installed on the sites and the ones without it. I'm not sure how far this is true, but personally I did see an improvement on installing their site widget.
- Alexa do not penalize websites whereas Google might.
- Alexa Rank is purely based on traffic whereas Google PR depends on its algorithm.
- Alexa Rank may vary from millions to as low as 1 whereas Google PR uses a floating number algorithm to rank a website on a scale of 10.
- Other factors that contribute to Alexa Rank apart from traffic - their toolbar.
- Other factors that contribute to Google PR apart from back-links - Page Speed and other unknown craps understood by SEO geeks.
To know more about PR & back-links, check out these:
Article One - How to increase the page rank of my blog?
Article Two - How to increase back-links to my blog???