Clone Mode is a Mode in which output of one Monitor is duplicated onto other Monitors. There can be many possible uses of Clone Mode.
You can setup Clone Mode for your Computer in two broad possible ways.
1) By Connecting a Dual Head Graphics or Multiple Head Graphics Card to you Computer.
2) By Using Display Switch to duplicate output on other Monitors or Display Devices Connected.
In case you use graphics card, you can have the possibility of extending display (i.e. span Desktop across Dual or Multiple Monitors) and can have full control of Dual Monitors connected to your Computer. However if you use Display Switch then you can only Clone output of your Computer to Multiple Monitors and the Display Switch is quite cheap as compared to Dual or Multiple Monitor Graphics Card. XP, Vista and Window 7 do support connecting Dual or Multiple Head Graphic Card and controlling them such as Switching to Extended Mode, Switching to Clone Mode, Enabling or Disabling Monitor(s) Connected to your Computer.