Recently posted on the Official Google Blog, Google is planning on rolling out an update for Gmail's inbox which changes the overall look of the inbox. This is largely an aesthetic update, but the default is a bit different from the interface we all know and use daily. The video below gives us a brief tour of the new inbox.
If you do not like the idea of multiple tabs in your email, you can always switch it back to classic mode.
This update should be rolling out over the next few weeks, though a specific time has not been stated.