have voltage directly analogous to the strength of the corresponding physical signal, that is, the loudness of a sound or the brightness of a light source. The voltage of the signal alternates [between positive and negative) at the frequency of the sound or light source.
Signals which come directly from the read/write heads, before digitalisation. No analog signals indicate that no data has been stored on the media, or that the data has been removed using a strong magnetic field such as a Degausser. It differs from a digital signal in that small fluctuations in the signal are meaningful.
A type of signal that encodes voice, video, or data transmitted over wire or through the air, and is commonly represented as an oscillating wave. An analog signal can take any value in a range and changes smoothly between values, as opposed to digital signals, which is characterized by discrete bits of information in numerical steps. An analog signal can transmit analog or digital data.
Signals which can vary over a continuous range (e.g., the human voice over conventional telephone lines). Analog circuitry is more subject to distortion and noise, but it is more capable of handling complex signals than are digital signals which can have only discrete values.
Audio and video signals currently used in broadcasting are analog as are many telephone lines. The signal is represented represented by a continuous wave because the signal is constantly varying within a range as opposed to pulsed, discrete digital signals. For a better description with pictures, see the section on the analog wave at Howstuffsworks.