Excerpted from Twitter Blog:
"The Discover tab makes it easy to discover information that matters to you without having to follow additional accounts. Starting today, the Discover tab will begin to surface content that is even more personalized and meaningful to you.
We’ve incorporated additional personalization signals to select Discover stories, including Tweets that are popular among the people you follow and the folks they follow.
The Discover tab’s new design shows who tweeted about particular stories. You can click “View Tweets" on any story to see popular Tweets from your network or recent, relevant Tweets directly below the story summary.
This social context helps you understand why each story matters to you and makes it easier to join the conversation. You can reply, retweet or favorite these Tweets, or you can “Tweet this story” to share your own perspective..."
Read full article here: http://blog.twitter.com/2012/05/discover-better-stories.html