A tool used by DJs - or disc jockeys - to mix audio signals outputted from turntables, CD players, casettes players, and various other audio sources. Typically, a DJ mixer has two input channels. Each channel is made up of a gain control, a volume slider, and optionally a balance control and equalizer.
Most DJ mixers have a cross-fader. This is used to fade between the two audio sources. For example, if the cross-fader is on the far left, channel one will be playing; if it is on the far right, channel two will be playing; if it is centered, both channels will be playing simultaneously.
DJ mixers are typically used at raves and dance clubs to mix anything from house music to popular music to hip-hop. The purpose of the DJ mixer is not to be confused with a studio mixer, which serves an entirely different purpose.