The W particle is one of the intermediate vector bosons, a pair of particles (the other is the Z particle) predicted in 1979 in order to unify two of the fundamental forces: the electromagnetic and weak forces into the electroweak force. It can be either positively or negatively charged. The theory states that although the electromagnetic and weak forces have the same strength the weak force is weaker because the exchange particles (particles wich transmit a fundamental force between particles), the intermediate vector bosons, are much more massive than the exchange particle of the electromagnetic force, the photon. At the time of the big bang, the two forces were the same , but as the universe cooled there was a spontaneous symmetry break, leading to the difference we detect today. The precise reasons for the large difference in mass is unknown.
They were detected at CERN in 1982 and the W particle's mass was measured at 82GeV.