The W and Z particles are called the 'vector bosons' and they are responsible for producing ( causing, mediating) the weak nuclear force. They are classified as bosons because they have integer quantum spin. The quarks which comprise neutrons and protons, and the leptons which include electrons and neutrinos, are classified as fermions because they have half-integer quantum spin.
Remember, bosons mediate or transmit forces, while fermions are all the other particles upon which they act.