New at Twitter : "Lists" beta released today !
Twitter introduced a new feature called “Lists” today and giving access for this beta feature for few accounts only. All users will get this feature after beta testing.
Basically Twitter “Lists” is a grouping feature ,helps you to group the list of the people you are following according to the category/interests .
Lists are timelines you build yourself, consisting of friends, family, co-workers, sports teams, you name it. You’re part of a small group receiving this feature, so don’t tweet about it yet!-Twitter
The lists feature is very useful and I have tested , working fine . When you organize the people you are following into groups, it is easy to identify the number of the people who are following back from the group .In the following illustration I have created a List called Mobile_Group.

I think its given to only 5% of the total users, here is a link – http://bit.ly/1ooxLA
Yes thanks for the useful information .