Google introduced a new feature in Gmail which makes it easier to read emails. It’s a lab feature called Preview Pane and when enabled, users can switch between regular list view in inbox or the newly introduced pane view.
The feature is more like what Hotmail/Live users get to see in their inboxes. Google first introduced Gmail two pane view on iPad (and other tablets) as an experiment. This feature is provided for regular desktop users.
Users can switch between different view modes with a toggle button in the top right corner in their Gmail inbox. Enable Preview Pane from Labs tab in Gmail Settings.