http://en.wikipedia.org/wiki/Audio_mixing:
Audio mixing is used for sound recording, audio editing and sound systems to balance the relative volume and frequency content of a number of sound sources. Typically, these sound sources are the different musical instruments in a band or vocalists, the sections of an orchestra, announcers and journalists, crowd noises and so on.
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An audio mixing console, or mixing desk, or mixing board, has numerous rotating controls (potentiometers) and sliding controls (faders which are also potentiometers) that are used to manipulate the volume, the addition of effects such as reverb, and frequency content (equalization) of audio signals. On most consoles, all the controls that apply to a single channel of audio are arranged in a vertical column called a channel strip. Larger and more complex consoles such as those used in film and television production can contain hundreds of channel strips. Many consoles today, regardless of cost, have automation capabilities so the movement of their controls is performed automatically, not unlike a player piano. A recent trend is to use a "control surface" connected to a computer. This eliminates much of the electronics in a conventional console as the actual mixing work is done digitally by the computer.
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