Tar - a resinous, usually brown or blackish substance.  Tar is the most generic term, because tars can occur naturally or by distillation or chemical reaction.

Bitumen - a black viscous mixture of hydrocarbons obtained naturally or as a residue from petroleum distillation.

Tars have higher specific gravity than bitumens and lower viscosity and these properties give them greater penetrating power. Thus we can say Bitumen is more viscous than tar pitch.