Dynamic range describes the ratio of the softest sound to the loudest sound in a musical instrument or piece of electronic equipment. This ratio is measured in decibels (abbreviated as dB) units.
Dynamic range measurements are used in audio equipment to indicate a component's maximum output signal and to rate a system's noise floor. As a reference point, the dynamic range of human hearing, the difference between the softest sound we can perceive and the loudest, is about 120 dB.
Compressors, expanders, and noise gates are processing devices that are used in audio to alter the dynamic range of a given signal. This is done to achieve a more consistent sound when recording or as a special effect (by radically altering the dynamics of a sound, thereby creating a sound not possible from the original source).