A differential output refers to the fact that each channel has a complement channel, i.e. Channel A and Channel A not. A differential line driver is used to help increase noise immunity (see What are the A not and B not Channels used for?). A differential line driver also allows you to sink or source more current than a Totem Pole output. A differential line driver will work both with a sinking or sourcing circuit. (see What is a Sinking or Sourcing input?) It can also help in increasing the distance in which a signal is transmitted.