Hadrons are massive particles with internal structure. There are two classes of hadrons: mesons (bosons) and baryons (fermions). Hadrons are composed of quarks, so they interact via the strong force. Leptons are light particles with no structure. All leptons are fermions. It is believed that leptons are fundamental particles(otherwise, there would be leptonic bosons); leptons are not composed of quarks, so they do not interact via the strong force.