Mac OS X Lion seems to have borrowed many features from iOS. ( Although apparently, not as many as Mountain Lion plans to ) One of these, is the new Mail look. I personally love this look and find it far more convenient and organized.
Mail on Lion has a neat two-column view where you can see everything together side-by-side as shown above. It now shows a list of all your messages in the sidebar to the left with the selected message in the viewing column to the right. You can also click the 'Show' button to make it a three column view in which the extra column would be a list of your email accounts and folders.
However, many may have trouble adapting to the new look ( or may simply prefer the classic look ).
To switch back to the classic Mail look, go to Mail's Preferences ( By pressing command-comma or going into the Mail menu )
Go to the 'Viewing' tab and check the first option: "Use classic layout"