Google has added a new feature in their Labs which allows a user to enable a preview pane in Gmail. Rather than the conventional way of clicking an email, going back to the email list and clicking another email, you can now quickly switch between mails using the new Preview Pane.
To enable Preview Pane, go to the Labs Tab. Scroll down to see the Preview Pane setting.
Now check the Enable box and click Save.
Gmail will now reload and you will be able to see the preview pane immediately.
You can either set the preview pane vertically as shown below or horizontally like in Yahoo Mail. To change the position of the Preview Pane, just click on the toggle button present on the top right corner of the screen and choose the desired position.