This tab in Gmail has been around for months now. It was reportedly added by Google in an effort to save people time, and reduce the amount of promotional, yet not-quite-spam, emails that we're all forced to see on a daily basis.
For internet marketers, this change was quite a shock, because it meant that, by default, your Gmail recipients would likely not see your emails, as frequently as before, or in some cases (for people that never look at the Promotions tab)...never.
Since this is still a "field trial", you have to sign up to see it. You can do that here: https://www.gmail.com/intl/en/mail/help/fieldtrial.html
Then, once you're accepted into the trial, you'll see a new "Switch to Grid" button inside Gmail, which allows you to see a Pinterest-esque presentation of your Promotions emails.
If you send any promotional emails, you should include at least one image, minimum 580x400 pixels. This will increase the chances that your email will be featured in the Grid View, and will help maximize your open rates.
Also, keep your sender name limited to 20 characters, and subject limited to 75 characters. This prevents them from being truncated in Grid View.
Last, but not least, make sure you have a verified Google+ Page for your company, so your logo will appear inside the Grid View, rather than just a gray initial.
Kane Miller from Winning.Email
Kane has a background in computer engineering, but now focuses on creating new and interesting web services. His passion for improving his own email deliverability rates was the driving force behind the creation of Winning.Email.