Black bars at bottom and top of screen.
A letterbox is usually seen when widescreen content is displayed on a standard size screen. You might see this for example if you view a widescreen DVD movie where the image dimensions are wider than would fit your television or display.
You might also see this if you import widescreen footage into a standard (4:3) iMovie project, or when viewing photos in iPhoto that are landscape orientation.
Black bars at left and right of screen.
A pillarbox is usually seen when standard dimension content is displayed on a wide screen. You might see this for example when watching an older TV show on a widescreen HDTV, or Cinema Display.
You might also see this if you import standard DV footage (4:3) into a widescreen (16:9) iMovie project, or when viewing photos in iPhoto that are portrait orientation.
Black bars on top, bottom, left, and right of image.
A windowbox is usually seen when widescreen content was manually letterboxed for standard size screen (4:3), but then viewed on a widescreen display. You might see this for example when watching a commercial for an upcoming film, or an older music video on a widescreen HDTV or Cinema Display.
You might also see this if you import footage that was manually letterboxed into an iMovie project. You can tell if the footage has a letterbox effect already applied by viewing the footage in QuickTime Player.