Twitter is creating its own ecosystem and hierarchy around the number of followers, list mentions, RTs, etc.
Twitter lists show how others label you, for example, "science-types", "medical-education", "bloggers" or sometimes just "random-people".
In a way, Twitter has put everybody to work on their search product since lists are a way to tag an arbitrary set of real time web feeds. http://bit.ly/3cfoEO
A screenshot of an early version of some of my Twitter lists.
According to @Doctor_V, "You can tell a lot about someone by the lists they keep."
Twitter: There's a List for That http://bit.ly/1Afe0