These are different video settings that you can choose to optimize your viewing. In mirror-mode (also known as clone mode), the external display is the same as (mirrors) what is shown on your laptop, i.e., whatever you see on your laptop screen, will be displayed on your external monitor. Extended-mode enables you to display different views on each display. For example you can surf on one display while showing a DVD on another display and have your email on the other display or you can spread an Excel worksheet across all 3 displays to see as many columns as possible.