A more social stream-centric approach to email; see An inbox that works for you (Google Blog) for more details
"“In 2004, when Gmail was launched we were still 3 years away from the first iPhone.” While the world has moved from clam-shells to touchscreens, email hasn’t evolved in step. Inbox is Google’s attempt to bring email up to speed. “We thought, ‘What if we started completely fresh, what if we built a whole new product that was an inbox that really, truly tried to do the work for you?’—an inbox that tried to help you get back to what mattered to you,” Gawley said of Inbox’s inspiration.Gmail’s New Inbox App Puts the Important Stuff on Top - Personal Tech News - WSJ
And indeed, Inbox does look different than other email apps. It looks more like a social networking feed than a traditional inbox. And that’s the point, said Jason Cornwell, the lead designer on both Gmail and Inbox. “It looks really different than Gmail, but that’s because we’re trying to solve a different problem,” Cornwell said. “We’re trying to be the best place to get back to the things that matter to you.”"