With a resistive touch screen panel, there are several layers to the actual screen. Two of these layers are very thin metallic layers that are situated very close together, but with a space between them. When a finger, hand or stylus of some sort is pressed into the screen, the two layers which are being run through with electrical current, are briefly pressed together, causing a larger current reading and allowing the controller portion of the apparatus to register that a touch has taken place and carry out the command. This type is most commonly found in cell phone touch screen panels.
Another type of touch screen panel is that of surface acoustic panels. These panels are not often used because of the easy with which interference or damage can be implemented on them. With surface acoustics, sound waves are flowing through the screen like the electrical current in the resistive type, and when the screen is touched it causes an interruption of the sound wave that is moving through it, registering that a touch has occurred. These screens are very particular and can often malfunction due to dust, dirt or contaminants on the screen and they are often easily damaged by rough handling or outside elements like other sound waves.
Capacitive touch screen panels are much more complicated than the resistive or surface acoustic panels. With capacitive panels, the electrostatic field that every human has reacts to one in the phone. The electrostatic field in the phone is designed by coating an insulating material with a conductive material, like glass with indium tin oxide. The phone, monitor or projector will pick up on the difference in capacitance and register a touch has occurred. With this type of touch screen panel, only a direct touch from a human body part can make the screen register that it happened, but these too are also easily damaged accidentally.
Optical imaging is a form of touch screen panel that uses cameras and shadows to register a touch on the screen. Cameras are placed along the edge of the screen and the entire screen is backlit. When the touch occurs, it shows up as a shadow between two of the cameras and the super computer inside the device triangulates the position of the touch and what icon it must've been pushing. For larger devices like laptops and computer monitors, and educational projectors, optical imaging is increasing in popularity.