With the old Facebook Messages layout, all of your messages were laid out in descending order (most recent at the top). When you clicked on a particular thread, you were taken to a different page where the message thread was displayed. Facebook has just announced that you'll no longer have to navigate back and forth between your inbox and your actual messages.
The side-by-side view is one of the improvement that's part of the newest update to Facebook messages. Now, your inbox will appear on the lef-hand side, and you actual message threads on the right. Everything on the same page - awesome.
Facebook has also added multiple photo functionality to messages, as well as emoticons.
Also coming along with this update: better search and navigation. You can now search by user name and keyword as well as utilize new keyboard commands.