A liquid crystal display touch screen has additional layers built on top of the display elements themselves, which provide the touch functionality.  There are many methodologies for sensing user touch on a display, but the two most common are resistive and capacitive sensing.  Among those two capacitive touch screens dominate the market, especially in cell phones.  This is because there is a smaller glare effect in the capacitive form, and more flexibility in multi-sensing options than in the resistive counterpart.

On top of the LCD display a non-conductive, transparent barrier is placed, which in many cases is glass.  Following this is usually the sensing elements, which in capacitive applications are able to detect voltage or current drops at specific locations and report them to the controller, often multiple locations can be independently determined.  On top of the sensors is the capacitive layer itself, which has very small lines holding a charge that can be affected by any conductive material touching the screen.  On top of this the standard protective layer is bonded in place.