The “new” Twitter released this month. It’s a significant overhaul, meant to make Twitter more intuitive for beginners. You can read about it on the official Twitter blog:
Today we introduce a new version of Twitter. We’ve simplified the design to make it easier than ever to follow what you care about, connect with others and discover something new. You’ll see this new design both on Twitter.com and mobile phones, so that you’ll have a familiar experience any time, anywhere. We’ve also updated TweetDeck to be consistent with this new version.
Mashable has a nice video explaining the changes.