There are two categories of damage, physical and elemental. In physical damage, swords are stronger than bows, bows are stronger than spears, and spears are stronger than swords. For elemental damage, fire and ice are neutral to each other, and lighting and darkness are neutral to each other. You will do more damage when facing an opponent that is weak to your physical and/or elemental damage. You can determine a character's physical type by the weapon icon at the top right corner of their box. Their elemental type is revealed in the bottom right corner (red for fire, blue for ice, yellow for lightning, and purple for darkness).