You could do this already by adding accounts to the Gmail app, but now you can view all your disparate inboxes in one steady stream. 

The app's new "All Inboxes" option collects all your messages in one place, and you can still see each inbox separately by tapping the bubble icons or using a drop-down menu in the sidebar. To add accounts, you hop into the updated app's Settings options, select "Add account," and choose from a simple menu that lets you pick between a Google Apps/Gmail account and an IMAP/POP webmail account. From there, just plug in your Yahoo, AOL, Outlook.com, Hotmail, or MSN.com account—whatever olden-times webmail services people used before Gmail.

This is not exactly a revolutionary development. After all, stock email apps on everything from the iPhone to the BlackBerry let you filter in all your email accounts in one place—with integrated support for Exchange-based work email, to boot. But the new Gmail app's secret sauce is that it brings elements of the Gmail interface to these external accounts.