Warcraft 3 provides each unit with a certain type of armor and a certain type of attack. This results in certain units countering other units. It is important to understand what type of unit has what attack or armor type in order to figure out how best to counter it.
The below image first shows what attack type and armor type each unit in the game has. The right side shows how much damage percent wise each attack does vs each type of armor type.
In game you will see all these icons showing what type of attack/armor each unit has. The hammer icon for “attack type” indicates spell damage. It does full damage against all units besides heroes which have 30% damage reduction against spells. Thus, if you coil a hero with a level 1 coil (which should be 100 damage) you will only deal 70 damage.