The @username tab replaces the mentions tab, and is more of a personal
notifications dashboard. The tab will now include tweets directed at users via @replies, highlight "favorited" tweets and retweeted tweets, and include new follower notifications. Meanwhile, the
Activity tab's purpose is similar to activity notifications that appear in Facebook's News Feed. The tab showcases the same actions -- retweets, favorites and follows -- as the @username tab, but as
they pertain to the individuals a user follows.
"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," the official Twitter Blog explained on Wednesday. "It's easier than ever to explore Twitter, connect with people, and discover what's happening around the world."