Preview Panes are those sections at the bottom of list pages that show more information about the selected record in the list page's grid. For a preview pane, you can use ANY type of part available in AX 2012 (Info Parts, Form Parts, Cues and Cue Groups). Info parts are best suited as a preview pane part. They are easily modeled in the AOT and work on both the rich client and Enterprise Portal. Cues and Cue Groups also work on both clients, however they are not your typical Preview Pane since they do not provide a list of data but rather a summation or count of records with a graphical representation. Form Parts work well as Preview Panes, however they have separate development paths for the rich client versus the Enterprise Portal. For use in the rich client, you develop a form in the AOT. For use on the Enterprise Portal, you need to develop a web user control.
When you test a preview pane, you may notice it shows up as a regular factbox on the list page. Generally, opening the list page form from the AOT seems to have some quirks. If you want to test a list page with its preview panes and factboxes, create a menu item. That way you can add it to the menu and open the list page in the client, or you can right-click the menu item and deploy the list page to the Enterprise Portal. If you are testing on the Enterprise Portal, you do not have to deploy the list page after making changes to it. You'll need to "Refresh AOD" to pick up any changes to your list page. You can find that in the quick launch menu in the "Home" module on the Enterprise Portal.