ALEXA dot com

I've been wondering on how Alexa works lately. When I'm reading one of my friend's post about alexa, I found out a comment from anonymous fellow describing briefly and clearly about alexa, this is his comment:

Alexa Internet, is a California-based subsidiary company of Amazon.com that is best known for operating a website that provides information on the web traffic to other websites.
Alexa collects information from users who have installed an “Alexa Toolbar,” allowing them to provide statistics on web site traffic, as well as lists of related links. It uses these data to form Alexa ranking. There is some controversy over how representative Alexa’s user base is of typical Internet behavior. If Alexa’s user base is a fair statistical sample of the internet user population, Alexa’s ranking should be quite accurate. A known source of bias is the self-selecting, opt-in nature of Alexa traffic tracking software installation, but the significance of this bias on rankings is not reported. Alexa ranking measures traffic ranking collected from Alexa toolbar users. High alexa ranking has no relationship with high google ranking, yahoo ranking, and etc.
Ranking in Alexa is significant as it can significantly increase your bargaining power when it comes to selling advertising on your site. If advertisers see that your site has a higher ranking, they are likely to think that they can sell more of their products and services by having a link on your site. This means that you can charge them higher fees for that link.