If you like having your messages list available even when viewing, this is a great feature to have. To enable it, just open Gmail, go to Labs, and turn on the Preview Pane option.
The Preview Pane lab works like a charm with all your beloved Gmail shortcuts (if you’re not familiar with them, hit the question mark [‘?’] in Gmail to bring them up). To turn it on in your Gmail account, head to your Gmail Labs page, Ctrl/Cmd+F for “Preview Pane”, select enable, and click the Save Changes button at the bottom of the page. If you don’t fancy the default layout, you can adjust the size of the panes with a little drag and drop.