An analog (or analogue, non-US spelling) signal is any continuously variable signal. It differs from a digital signal in that small fluctuations in the signal are meaningful. Analog is usually thought of in an electrical context, however mechanical, pneumatic, hydraulic and other systems also use analog signals.
An analog signal uses some property of the medium to convey the signal's information. For example an aneroid barometer uses rotary position as the signal to convey pressure information. Electrically, the property most commonly used is voltage followed closely by current and charge.
Another method of conveying an analog signal is to use modulation. In this, some base signal (eg a sinusoidal carrier wave) has one of its properties altered: amplitude modulation involves altering the amplitude of a sinusoidal voltage waveform by the source information, frequency modulation changes the frequency. Other techniques, such as changing the phase of the base signal also work.
Any information may be conveyed by an analog signal, often such a signal is a measured response to changes in physical phenomena, such as sound, light, temperature, position, or pressure and is achieved using a transducer.
Analog circuits do not involve quantisation of information into digital format. The concept being measured over the circuit, whether sound, light, pressure, temperature or an exceeded limit, remains from end to end.
Clocks with hands are called analog, those with digits digital. However a traditional analog clock is actually digital since the hands do not move in a smooth continuous motion, but in small steps every second or half a second.
The word "analog" implies an analogy between cause and effect, voltage in and voltage out, current in and current out, sound in and frequency out.
For example, in an analog sound recording, the frequency of a pure tone striking a microphone creates a corresponding variation in the current passing through it. An increase in the volume of the sound causes the fluctuation of current to increase while keeping the same rhythm.
The primary disadvantage of analog signalling is that any system has noise in it -- that is, random variations. As the signal is copied and re-copied, or transmitted over long distances, these random variations become dominant. Electrically these losses are lessened by shielding, good connections and several cable types such as coax and twisted pair.
The effects of noise make signal loss and distortion impossible to recover, since amplifying the signal to recover attenuated parts of the signal amplifies the noise as well.
See digital for a discussion of digital vs. analog.