Google offers a number ranging from 0-10, in the order of increasing importance, to all websites. An extremely good impressive website will have a PR 10 and a site which has not enough content, backlinks or a new site will have a very low page or 0 rank.
The way how Google calculates and assigns PR to a site is not well known, but there are some factors that contribute to that. Some of which include great content, enough content, quality backlinks, traffic, relevant and quality outbound links, the age of the site and so on.
Unlike Google’s page rank, Alexa ranks a website based on its traffic. It is actually called Alexa traffic rank. Alexa takes care of the reach of your website and the number of quality backlinks too. Further, having a good Alexa rank will help you to get advertisers for your blog/website