The Gmail app is hugely popular and not about to go anywhere, so what exactly is Google aiming for with Inbox? This new invite-only email app is like Gmail mixed with Google Now: messages are automatically grouped into bundles based on category, eg shopping, travel, bills, etc. Plus you can create your own custom bundles. There's also an integrated to-do list sitting at the top of your inbox and you can "snooze" both these and your emails in order to deal with them at a later date. While this sounds like it will help most of us be a little bit more productive, it doesn't currently work with Google Apps for Work accounts, which is a pity.

google.com/inbox