Twitter on the web got some semi-major changes. The interface is virtually the same, what's changed is where things go. For one, they've included a Facebook like Newsfeed under “Activity”. The tab for “Activity” is placed where the old tab for “Retweets” used to be in the central bar. In “Activity”, you will literally see the activities of people you follow, not including their tweets. What this means is if someone you follow starts following someone else, it will show up here. If someone you follow retweets someone, it will show up here. If someone you follow even favourites another tweet, it will show up here. In order to promote “Activity”, they've also included an “Activity” box in the right hand side box, where the latest three 'activities' show up.
Notice the 'Activity' tab. It's new.
On to the second change. @Mentions in the tool bar has changed to @yourhandle. So for instance, my @Mentions has changed to @paddychop. This is because, not only do you receive your mentions here, you also see your tweets that have been retweeted by other Twitter users here. With this change, you can finally see who retweeted your tweet, a function that was faulty when the new Twitter was launched. You will also see new followers i.e. people who have just started following you, here. You can select to receive only @mentions, here by checking a box, or you can stick to retweets and follows.
The new changes haven't seemed to have been rolled out to everyone, yet. However, once it is rolled out to you, Twitter will place a little black box pointing to the center bar on your homepage, alerting you to these changes. Until then, sit tight.