The People app can be viewed as an aggregator for all your social networks, or as Microsoft puts it, a 'fully connected address book'. That's because when you connect your Microsoft account to services like Facebook, Twitter, and LinkedIn, your contacts from these networks will also show up in the People app. For the nerds among us, this is possible by way of standards and APIs such as Exchange ActiveSync (EAS), OAuth, and REST. Through the People app, you can quickly drop your contacts an email, see their status updates, and comment on their social activities and photos. Choose 'What's new' to see a summary of the latest happenings across your social networks. You can also drill down to a single contact, and see what he or she is up to lately. But what if you've the same person listed in two or more different networks? The People app will try to link them up, and present them in a single contact card.
Again, don't be mistaken that only the built-in apps like Mail and Messaging have access to the contact list. Other apps too can take advantage of it too. For example, since the People app supports the Share 'contract' (as a target), if you were to chance upon an interesting article in Internet Explorer, you can share it on Facebook or Twitter through this app that's listed in the Share Charm.
All in all, the People app is the best Modern-style app on Windows 8 by Microsoft so far.