A desk is a mixing desk, like one of these !

It has a channel strip for each mic input. Typically each channel strip will have a gain (small amplifier) at the top, followed by a 3 or 4 button EQ, followed by an effect send button followed by a Pan and a volume slider at the bottom.

Net end result is that you can tweak each channel coming into the desk turning it up or down to balance everything / add effects as you want. This means that if some one in the band has a louder singing voice than someone else they can be brought up to a level that can be heard.

you might want to look at one of these also

and this section also