Once Activated, Gmail will begin to automatically tag certain inbound messages as ‘Bulk’, ‘Notifications’, or ‘Forums’ and handle them accordingly. Notifications includes updates from Facebook, Twitter, Amazon shipping confirmations and other messages that may not be from someone you know but contain important information. And Forums consist of messages from any online message board you may take part in. Messages that appear to be sent as part of a mass mail list will probably be marked as ‘Bulk’ — by default these will skip the inbox, while Forums and Notifications will be kept in the inbox by default.
To enable the new feature, go to Gmail’s Labs section, Search for ‘SmartLabels’ in it and enable the same. Smart Labels is still optional under Labs for the time being.