If an Attack Position monster is attacked, the monster with lower ATK is destroyed by battle and its controller takes battle damage equal to the difference in ATK. If their ATK is equal, both monsters are destroyed.
If a Defense Position monster is attacked and the attack target's DEF is less than the attacking monster's ATK, the attack target is destroyed by battle, but neither player takes battle damage (unless the attacking monster inflicts piercing battle damage).
If a Defense Position monster is attacked and the attack target's DEF is greater than or equal to the attacking monster's ATK, then the attacking monster's controller takes battle damage equal to the difference between the attacking monster's ATK and the attack target's DEF, but neither monster is destroyed.