Analog Signal: an analog signal is a continuously varying signal, often used to represent sound or image information. The signal's amplitude can smoothly change over time, as opposed to a digital signal which has only two states (on or off). Digital Signal: a digital signal is a discrete signal that represents data using two voltage levels, often called "0" and "1". Output signal: an output signal is a type of electrical signal produced by an electronic device to control or interact with another device. Electrical signal: an electrical signal is a type of energy that propagates through a conductor (such as a wire) as an electric current. Sound signals: sound signals are analog signals that convey information about sound. Sound signals can represent anything from the simple sounds of everyday life to the complex soundtracks of movies and television shows.