Capacitive Touch Screens

    Using above the screen of many cellphones is a thin capacitor used to sense your finger's position. In the iPhone, made by Apple, the capacitors are arranged in a coordinate system. This allows for a matrix of measurable capacitors. Multiple "touches" can be detected to a high accuracy because of this.
    Mutual Capacitance requires two separate layers of material. One line is subject to a current while the other is used to complete the capacitor and to measure the voltage.
    Self Capacitance uses an outside conductor such as a finger. The plates in the phone are subject to a voltage and have capacitance-sensing circuitry. A dielectric such as glass is between the sensor and your finger.
    When you touch the phone, a series of values are given to the processor. These are measured by the individual points as well as the surrounding points but these are likely to have a lower capacitance.

                           http://electronics.howstuffworks.com/iphone2.htm

    In recent development, capacitive sensors have actually been integrated into the screens so that the screen and pixels themselves act as a conducting plate. The thin glass overlay acts as a dielectric and though a system of volt meters, can triangulate your finger as it comes close and touches the screen.

http://scienceline.org/2012/01/okay-but-how-do-touch-screens-actually-work/

3D Capacitive Sensor (Super Cool STUFF)