made of plastic

All buttons except Home were made of plastic on the original iPhone and metal on all later models.

 The touch screen furnishes the remainder of the user interface.

The display responds to three sensors (four on the iPhone 4).

 A proximity sensor deactivates the display lights and touchscreen when the device is brought near the face during a call.

 This is done to save battery power and to prevent inadvertent inputs from the user's face and ears.

 An ambient light sensor adjusts the display brightness which in turn saves battery power which is an effective function for your phone.

 A 3-axis accelero meter senses the orientation of the phone and changes the screen accordingly, allowing the user to an easily switch between portrait and landscape mode.