After the release of Google+, Google's rolling out its preview of Gmail's makeover. They've been working on interface updates that allow Gmail to become more aesthetically pleasing as its features have also increased over the years. Google said on the Gmail blog that they're not rolling out the new features soon, however, the new interfaces will roll out over the next few months. Today you can see the "Preview" and "Preview Dense" themes in the themes tab in Gmail settings. The new interface will eventually expand to accomodate different screen sizes and user preferences, which is why there are two themes, and until then users can pick the information and density they prefer.
The new cleaner interface
The preview shows a cleaner and a more 'modern' look to Gmail, however some features still aren't redesigned to live up to the new aesthetic standards, like Google Labs. The new themes make it look a little strange, according to Google, and they're working on smoothening the rough edges. As these themes roll out, users can give Google (and us in the comments section) feedback on the new look. Google Calendar is also getting a redesign, however, users don't need to turn that on. The new look will automatically update in the next few days.