The design has been updated to feature a four-tab system that brings “you instantly closer to everything you care about.” The four categories are Home, Connect, Discover, and Me.
Home is where you can view tweets, pretty much like before. Connect is where you can see who follows, mentions, retweets, or favorites you. Discover is a place to find out about the latest trends and hashtags. Finally, Me is where you can manage your profile.
Also, Twitter has implemented the Tweet button on a constant toolbar at the top of the interface so it is easy to interact with the world from wherever you are within the application.
Eventually these changes will be seen on Twitter’s main page, but for now, they are included on the mobile site, Twitter for iPhone, and Twitter for Android. You can update your software right now to try out the new design.
source: Twitter Blog