If you're one of those lucky people in the Twitter beta testing group, you would know that there was an update tonight which has a new user interface. Twitter has changed the basic interface of its app, making it more simple to use.
The left edge slide-in drawer is gone, and the scrolling tabs at the top of the page have been replaced by a simple three tab design: Home, Discover and Activity. Notifications and messages have been moved, which now feature their own distinct buttons in the top action bar, with number badges indicating unread counts.
If you want to compose a new tweet, that's now at the bottom of the app, where you'll also be greeted with new buttons to add a picture, launch into the camera, or directly into the tweet and start typing.