Twitter is in the process of rolling out new features that give a more in-depth look at the activity of the people you follow, as well as how people are interacting with you. The new Username tab combines the older Mentions and Retweets tabs, and provides a more streamlined way to see when you're mentioned, or when your tweets are favorited and retweeted; it'll also give you a place to easily track new followers so you can decide if you want to follow them back. The downside for those who get a lot of mentions, retweets, and followers is that you'll have a harder time picking out mentions that you might want to reply to from the stream of information.
As for the Activity tab, it's all about what the people you follow are doing, besides posting tweets. Here you'll see favorites or retweets by the people you follow, as well as when those users follow others, all with the goal of providing a more robust way to find new people to follow. While Twitter announced these features in August, they haven't appeared to roll out on a wide scale until now. For the moment, these features are only available via the website; while they'd be welcome additions to Twitter's mobile apps, those apps haven't always gotten the most timely or feature-complete updates. We'll be hoping these streams move beyond the website before long.