Somebody else's song. A nascent band employs these to pad out their set list; an up-and-coming band will use the familiarity of one in order to reach a wider audience: e.g., Talking Heads with Al Green's "Take Me To The River", Run DMC with Aerosmith's "Walk This Way", or Marilyn Manson with Eurythmics' "Sweet Dreams". Some bands - cover bands - play covers exclusively, though this is a somewhat dubious practice.