Inbox by Gmail is Google's latest attempt to re-invent the way we deal with email. They're billing it as "an inbox that works for you." We get more email than ever, and Inbox helps organize and mine all that email for information so you spend less time maintaining your email and more time actually doing things with it. It does this a couple of ways.
Bundles - you guessed it - bundles emails together, going a step beyond the familiar categories we've enjoyed in Gmail in recent years. It'll bundle online receipts and the like, and will learn from your habits and your email over time to better bundle emails.
Highlights have morphed out of the 'Important' tab of your Gmail inbox, highlighting information from said important emails, and even adding in Google Now-like information, like real-time status on flights and packages. This helps you find the information you need fast so you can get in, get out, and get on.
More Google Now-like integrations are here in Reminders, which can be added to emails to ensure you get to them when you need to. You can also add to-dos, called Assists, to help get things done with helpful info. You can snooze these helpful hints if they come up at a bad time.
Right now Inbox is invite-only, with invites only coming from Googlers and from firstname.lastname@example.org, but that should be changing before long.