There will be 3 types of damage in the game: physical damage, magic damage and pure damage.
Physical Damage: Physical damage can be mitigated by armour.
Magic Damage: Magic damage will always fall into one of the following 3 subtypes: fire damage, frost damage and shock damage. Magic damage can be mitigated by magic armour and by subtype specific magic armour.
Pure Damage: Pure damage cannot be mitigated in any way. The amount of pure damage that an attack or spell deals will always be the amount of pure damage an enemy takes.
Resistances are Stats that make a unit more durable.
Armour: Reduces physical damage taken.
Magic Armour: Reduces all magical damage taken.
There are 3 subtypes of magic armour that reduce type specific magic damage.
Fire Armour: Reduces fire damage taken.
Frost Armour: Reduces frost damage taken.
Shock Armour: Reduces shock damage taken.
Armour penetration and magic penetration increase the damage the attacker deals by reducing the defender armour and magic armour.
Armour Penetration: Reduces armour of the defending unit.
Magic Penetration: Reduces magic armour of the defending unit.
There are 3 subtypes of magic penetration that reduce the defending unit magic armour when the damage being dealt is of that subtype of magic damage.
Fire Penetration: Increases fire damage dealt by reducing the magic armour and fire armour of the unit being attacked.
Frost Penetration: Increases frost damage dealt by reducing the magic armour and frost armour of the unit being attacked.