Google knows a lot about you, but it doesn't know everything. It can guess at common human behaviors, like how you tend to email the same people over and over, and only give cursory glances to newsletters and updates. But it doesn't know that an email from a recruiter you would never expect to see could be the most important thing in your inbox.
Google has previously implemented Smart Labels to catch bulk-style mailings and app notifications, and Priority Inbox to learn which messages might be most important to you. Now, with an upcoming update to Gmail on desktops and mobile apps, Gmail is scaling back from "We know what you like" to "Here's everything we can't quite categorize, some of which may be important."
That is: there are still "Smart Labels," but they now exist as viewpoint tabs, mounted on the top of your inbox. You can look at all your Social, Promotions, Updates, and Forums messages, or check out Primary to see what couldn't quite be categorized. It's an interesting step sideways, if not quite back, from pretty much everything else Google is aiming to do in the realm of predictive, contextual computing: Google Maps that know what kinds of routes and things you care about. Google voice searching that understands pronouns ("Where is Timothy's Coffee? How far is that from here?"). But it makes sense, because the stakes in email are a little bit higher than a flubbed search or a weird-looking map.
I haven't yet received access to the new labels on desktop or mobile Gmail, but from the look of things, it seems like a subtle step forward in email filtering, but a big design jump for Gmail. Gmail has always been about left-side controls that show you messages tagged/labeled in certain ways. To put "tabs" on the top of the inbox turns Gmail from less of a search-based tool and more of a traditional inbox, one slightly augmented by Google's algorithmic message categories.