Google PR: importance of page based on the number and quality of inbound links to your site.
Alexa: based on the traffic on your website as calculated by the Alexa toolbar

Google PR: rank 1-10. Higher it gets, its better for your site
Alexa: count of 1 million, little gets better for your website

Google· Pagrank

Google Pagrank And Alexa rank

Google PR: importance of page based on the number and quality of inbound links to your site.
Alexa: based on the traffic on your website as calculated by the Alexa toolbar

Google PR: rank 1-10. Higher it gets, its better for your site
Alexa: count of 1 million, little gets better for your website