Stylus
A small, pen-like instrument used to "write" on a touch-screen device.
A stylus provides far more accuracy than could be achieved with a fingertip, and provides a familiar pen-like experience for inputting handwriting or drawing.
A stylus can be nothing more than a simple stick of plastic, except that its tip is specifically designed to not damage the screen.
While any stylus should work with a resistive-type touch screen, compatibility can be an issue with the more-common capacitive type of touch screen. Either the stylus must be designed for capacitive displays, or the display must have an extra sensor for a specific type of stylus.
More advanced styli may have one or more buttons, and some include tip-pressure and/or motion sensors. Some include Bluetooth so they can double as a remote control.
See: Touch Screen