Heavy weapons are self explanatory. Really heavy stuff. Like gigantic clubs. When they are swung, they cover a huge arc infront of you and not only often one-shot opponents but they also smash through their limbs like butter.. If they aren't blocked.
They are however, insanely heavy and one of such decent weapons can weigh up as much as to 80 kilos, whereas a newbie character can only carry about 20 in total without armor.