Touchable plastic surface
This is most likely a capacitive touch interface.

This by making use of the rule that all circuit elements have some amount of capacitance, be it a capacitor a resistor or even a piece of wire.

The capacitance of these pieces is also weakly affected by certain materials, usually somewhat conductive themselves such as pieces of metal, wires or you.

There are a number of different ways to measure this but the easiest is measuring how long it takes to charge and discharge the capacitor this creates.

More information can be found on the Arduino website, see link below.

http://playground.arduino.cc/Main/CapacitiveSensor?from=Main.CapSense