DJs use equipment that can play at least two sources of recorded music simultaneously and mix them together. This allows the DJ to create seamless transitions between recordings and develop unique mixes of songs. Often, this involves aligning the beats of the music sources so their rhythms do not clash when they are played together. DJ equipment also allows the DJ to preview a source of recorded music in headphones before playing it for the audience. Previewing the music in headphones helps the DJ pick the next track they want to play and cue up the track to the desired starting location so it will mix well with the currently playing music.
The title "DJ" is also commonly used by DJs in front of their real names or adopted pseudonyms or stage names as a title to denote their profession.