Mixing records is a technique used by DJ's to ensure a seamless flow of music, without pauses which allows the build-up of an atmosphere, the vital 'groove' of a night's entertainment.
The DJ starts one track playing, and listens to a second track through headphones. Thanks to modern technology, DJ's can speed up or slow down tracks in order to synchronise the beat until it matches the track playing to the audience. The DJ then increases the volume of the second track until it reaches that of the first, and the two play simultaneously. At a suitable time, the DJ lowers the volume of the first track, allowing the second track to continue. This is called 'fading'. Modern DJ's can mix two, three, or even four tracks in this way, building a seamless series of tracks with no disernable break in between. The skill is knowing which tracks will mix together, and the sequence in which to mix them. The other method of moving between tracks involves finding a natrual break in a track, perhaps between beats, or at the end if the track stops rather than fades, and switching the second track volume up to continue to rhythm already set up. This is called 'chopping' and most DJ's will use a combination of the two methods.