The main reason I use to read Twitter is that it, unlike most other Twitter clients, lets me create groups of users, so I can separate out the people whose tweets I wish to read relatively quickly from headline tweets (such as from  and ) and posts from people I know more peripherally. Arguably, I could just follow fewer people, but at different times, it's useful to have quick access to my less-important groups.
In what is perhaps the first major change to the service since the beginning, Twitter has started rolling out, which lets you create groups of users via Twitter's Web interface, collecting together all the tweets from the people in the group. Lists can be either private, at which point they're just for your reference, or public, so anyone can see the tweets collected in them.
I see private lists as an official way of doing within Twitter what I do already in TweetDeck - organizing groups of people I follow as a way of limiting how much I read at any one time. If you've found Twitter overwhelming, try making a group of the people you find most interesting and focusing on that list most of the time.
Public lists, on the other hand, will serve as a way for people new to Twitter to find additional interesting people to follow (check out 72 lists, despite having only about 1,400 followers., for instance, or ). Public lists will also probably provide a more accurate way of determining influence within the Twitter network, since adding someone to a list is a significantly more intentional act than following them. Right now, I'm at around 3,650 followers and have been included in , but the TidBITS Twitter feed is already in
Twitter is also releasing a Lists API, so Twitter clients like Twitterrific and Tweetie will have to figure out the best way to integrate Lists into their interfaces. It's none too soon, frankly, since although TweetDeck offers group functionality, it's clumsy and incomplete (you can make groups, but you can't make a group of people not in any other group). Hopefully Twitter will build such a feature into Lists, along with making it easier to add people to lists directly from a tweet.
Not all users have access to the Lists feature yet, since Twitter is rolling it out in waves, but with luck it will be available to everyone soon.