Usually, an amplifier or simply amp, is any device which changes, generally increases, the amplitude of a signal. The connection of the input to the output of an amplifier-generally expressed as a function of the input frequency-is termed as the transfer function of the amplifier, and magnitude of transfer function is called the gain.
In common use, the term generally explains an electronic amplifier, where the input "signal" is generally a voltage or a current. In audio applications, amplifiers drive the loudspeakers employed in PA systems to create the human voice louder or play recorded music. Amplifiers may be categorized as per to the input (source) they are designed to amplify (likes a guitar amplifier, to perform with an electric guitar), the device they are intended to drive (like a headphone amplifier), the frequency range of the signals (Audio, IF, RF, and VHF amplifiers, for instance), whether they invert the signal (inverting amplifiers and non-inverting amplifiers), or the sort of device employed in the amplification (valve or tube amplifiers, FET amplifiers).
A related device which emphasizes conversion of signals of one type to another (for instance, a light signal in photons to a DC signal in amperes) is a transducer, a transformer, or a sensor.