Gmail tabs is a new Gmail layout rolled out in summer of 2013. By default it offers people the option of grouping incoming messages into "Primary," "Social," and "Promotions" tabs - "updates" and "forum" tabs can also be added.
Primary is for personal messages, Social for social media notifications and Promotions for marketer messages.
The introduction of Gmail tabs has beeen perceived by marketers as a new threat for email engagement as the promotion tab may be seen as low priority and secondary folder by users. It has also been perceived as a new deliverability issues, event though, an email in the promotions tab must be considered as a delivered message.
After the new tabbed view was implemented, email service provider MailChimp found that open rates dropped to 12% from an open rate of 13% but ESP numbers diverged on the subject.
Gmail users can actually drag a marketing email into their Primary inbox. In this way subsequent messages from the sender will be placed in the Primary tab. It is a new goal for email marketers.
Gmail tabs on Android:
How Groupon manages the Gmail Tabs issue: