From the website:
We know that “A-List” Twitterers like Scoble, LeoLaporte, and BarackObama have a lot of influence on Twitter, because they have tens of thousands of followers. However, social network analysis teaches us that there is a “horizon of communication” that extends beyond your own direct contacts, and this is demonstrated whenever somebody “retweets” a message. The significance is that not all followers are equal.
Rankings are calculated based on ‘Reach’, ‘Velocity’, and ‘Social Capital’.