From their blog:
Today, we’re rolling out two new features on Twitter.com that help you discover more on Twitter. You can now see when someone favorites or retweets one of your Tweets. You can also learn which Tweets are most interesting and inspiring to the people you follow.
The first new feature offers a simple way to see what's happening on Twitter in relation to you. Just click on the new tab with your @username, and you’ll be able to see which of your Tweets are Favorites, plus the latest Retweets (of your Tweets), Tweets directed to you, and your new Followers.
The other new feature is the Activity tab. It provides a rich new source of discovery by highlighting the latest Favorites, Retweets, and Follows from the people you follow on Twitter – all in one place. It’s easier than ever to explore Twitter, connect with people, and discover what’s happening around the world.