A stylus pen, a digital input device shaped like a pen, can function with two different types of systems, resistive systems and capacitive systems. In both cases, the user handles the stylus in the same way, but the components that sense the input are different.
In a resistive system, when a stylus pen presses on the screen, the two layers of the screen make contact with each other and disruption to the electrical field is recorded by the computer. One layer is electrically conductive, and the other layer is metallic and resistive.
In a capacitive system, the stylus disrupts the current in an layer of the screen that holds a continuous electrical charge. Sensors in the corners of the screens detect the change and send the coordinates of the disruption to the computer.
Capacitive systems have brighter, clearer screens than resistive systems because they transmit more light, but they only register input from conductive sources, such as a finger or specially designed stylus. Capacitive screens cost more to produce but are considered to be more accurate and responsive than resistive screens. Additionally, a stylus designed for capacitive screens does not work on resistive screens and vice versa.
iPhones use capacitive touch screens that have been modified to accept multiple input points on a screen. This modification prevents the use of any form of stylus on iPhone screens.
Surface acoustic wave systems are another form of touch screen, and are considered to have the best picture, but they do not work with a stylus.