The Alexa rankings are a ranking of websites based on visitors to the websites from those who have the Alexa toolbar installed in the web browser. The rankings do tend to be biased toward more tech orientated sites, as they are the ones who have the toolbar installed.
http://www.searchengine-faq.com/do-i-need-a-good-alexa-ranking/ (Search Engine FAQ)
http://www.searchengine-faq.com/how-to-improve-the-alexa-ranking/ (Search Engine FAQ)
http://www.forumfriday.com/seo/alexa/ (Forum Friday)
http://answers.yahoo.com/question/index?qid=20130526215617AA6r1BY (Yahoo Answers)