There are many different touchscreens. Resistive, Capacitive, Infrared, Surface Acoustic Wave, Near field imaging, and Light pens. However, I am going to talk about Resistive, since that is the most popular in touchscreen technologies. They basically have a flexible upper layer of conducting polyester plastic bonded to a rigid lower layer of conducting glass and separated by an insulating membrane. When you touch the screen, you force the polyester to touch the glass and it will complete a circuit in the touchscreen. A chip inside will figure out where you touched the phone and send out an output.
