The Z particle is one of the intermediate vector bosons, a pair of particles predicted in 1979 in order to unify two of the fundamental forces: the electromagnetic and weak forces into the electroweak force. The Z particle has no electric charge, but is otherwise very similar to the W particle.
The precise reasons for the large difference in mass is unknown. They were detected at CERN in 1982 and the Z particle's mass was measured at 93GeV.