When a finger (or specialised input device, such as stylus) comes in contact with the display it detects when and where on the display the user touches it. As a result capacitive displays can receive accurate input from very light touches.
Capacitive touch screens are built with a thin layer of conductive material, such as copper or Indium Tin Oxide (ITO), printed onto the underside of the display's insulating outer layer. When a finger touches the screen, a small electrical charge is drawn to the point of contact, which effectively becomes a functioning capacitor. The resulting change in the electrostatic field is then measured to pinpoint where the contact took place.