W bosons are, together with the Z0 boson, the mediators of the weak force, which is the only fundamental force that converts different leptons and quarks into each other, for example an up quark into a down quark or an electron neutrino into an electron.
It is called weak, but it is nonetheless very important! The W bosons mediate radioactive decays (beta decays), in which atomic nuclei achieve greater stability by converting one of its protons into a neutron (or vice versa). The same process is also relevant to the nuclear fusion that happens in the sun.
The weak interactions are divided in neutral and charged current processes, mediated by the neutral Z0 bosons and the charged W+ and W- bosons, respectively.
The W bosons were predicted in the so-called unification of the electroweak force. This theory, developed in the 1960s, states that the weak and electromagnetic interactions can be described as two aspects of the same force at high energies. Similarly, the theory predicts a Z0 boson that behaves exactly like the photon at high energies.
The W and Z0 bosons were discovered in 1983 by the UA1 and UA2 experiments at CERN with the Super Proton Synchrotron (SPS).