Google Page Rank and Alexa ranking are two different ways determine your site's performance. Here is a little description of the two
Alexa is one of the oldest and most popular traffic ranking tools widely used by webmasters, bloggers, and Internet Marketers. Alexa Rank is the rank of online traffic to a particular website, in comparison to all other sites on the web.
PageRank (PR) is a quality metric invented by Google's owners. The values 0 to 10 determine a page's importance, reliability, and authority on the web, according to Google. Yet PR is not important for any website anymore.