A while ago it was announced that Google would release a new and redesigned Gmail. This new version would besides a modern visual style also include features like actions on hover, a new sidebar and features available on Inbox like snooze and smart reply. This new Gmail is releasing today, and has already started rolling out to users.
If you are one the users who has recieved the new Gmail, you can activate it by going to the settings menu on the right side of the interface and press "Try the new Gmail".
Read more at The Verge.