Great weapons are especially large and heavy weapons wielded with both hands. This includes great hammers, great axes, two-handed swords and suchlike. A blow from a great weapon can cut a foe in half and break apart the thickest armour. It takes a long time to learn how to use these weapons and even then only extremely strong men are able to wield them effectively.
Note: Great weapons give +2 Strength in the first round of close combat, and +1 Strength in following rounds.