Damage types

There are several different categories of damage that can be inflicted. The different types of damage may be affected differently by modifiers. These types are magical, physical, mixed, pure, and direct HP removal damage.

Physical
Physical damage is caused by all regular attacks from all units, and only by certain skills. Physical damage is further divided into categories that modify damage based on the target. It is modified by both Armor and Damage Block, is unaffected by Magic Resistance or Magic Immunity, and cannot affect Ethereal units. Examples of physical damage spells are Juggernaut's Omnislash and Slardar's Slithereen Crush.

All physical damage is affected by both Armor and damage type. Example: a Plague Ward with 100 damage attacking a hero with 10 armor would do 100 * .5 for armor type * .625 for armor value (37.5% reduction) or 31.25 total damage.

The only exception to this is damage from Cleave, which is reduced by armor type but not by armor value or damage block. Practically speaking this means armor is worthless against cleave damage. Splash (from ranged attacks) is reduced as normal.

Hero Attacks
Hero damage is one subset of physical damage and does normal damage (100%) against everything except buildings and siege creeps, which take only half damage (50%) from heroes. Physical damage from hero abilities (such as Slardar's Crush) count as hero damage.

Normal Attacks
Normal physical damage (caused by most creeps) deals reduced (75%) damage to heroes and to buildings (70%), but has slightly enhanced damage (varies) to most neutral creeps.

Piercing Damage
Piercing attacks, such as Venomancer's Plague Wards, do reduced (50%) damage against heroes but enhanced (150%) damage to Lane Creeps, since they have light armor, making piercing damage more effective at lane control than hero harassment. Piercing damage only does 35% damage against buildings.

Siege Damage
Siege attacks, from siege Lane Creeps and Towers, do enhanced (150%) damage against Buildings and siege creeps but reduced (75%) against heroes.

Chaos Damage
The Chaos attack type is effectively the same as the hero type (does 100% to all), but does slightly less (40%) against the fortified armor on Buildings and siege creeps. Chaos is done by Invoker's Forge Spirit, by Roshan, the Fountain, several Neutral Creeps and Warlock's Infernal.

Magical
Magical damage is caused primarily by spells, although not all do magic damage. Magic damage is reduced by Magic Resistance, does not affect magic immune units, and does 140% against Ethereal units. Examples of magic damage are Broodmother's Spawn Spiderlings, Leshrac's Split Earth and Beastmaster's Primal Roar.

Pure
Pure damage is a form of magical damage that is unreduced by Magic Resistance or magic amplification (it always does exactly the damage it says it does). Because it is magical, it also ignores all Armor or forms of damage block. It does not affect units under Magic Immunity and does normal damage against Ethereal units. Examples of pure damage are Omniknight's Purification, Pudge's Meat Hook, Bloodseeker's Rupture, Spectre's Desolate and Dispersion, Enchantress' Impetus, Chen's Test of Faith, Silencer's Glaives of Wisdom, Tinker's Laser, Bane's Brain Sap, Timbersaw's Timber Chain and Chakram, Templar Assassin's Psi Blades, Invoker's Sun Strike and Outworld Devourer's Arcane Orb.

Composite
Composite damage (also known as Mixed Damage) is a physical damage that is also reduced by Magic Resistance. It is reduced by Armor, Magic Resistance, and Damage Block, but goes through Magic Immunity. It does not affect Ethereal units. Examples of composite damage are Beastmaster's Wild Axes, Alchemist's Acid Spray and Leshrac's Diabolic Edict (Diabolic Edict is not reduced by Damage Block).

HP Removal
HP Removal works by directly subtracting hitpoints from the affected unit. It ignores all damage reductions and affects units under Magic Immunity and Invulnerability and does normal damage against Ethereal units. Unlike all other damage types, HP Removal does not count as being attacked, so it will not interrupt Bottle or trigger any other on-attack effects. Examples of HP Removal are Huskar's Burning Spears (Health Cost), Vengeful Spirit's Wave of Terror, Bane's Nightmare, Necrolyte's Heartstopper Aura and Warlock's Fatal Bonds.

Universal
Universal damage is very similar to magical damage. It is reduced by magic resistance and deals 1.4x damage to Ethereal units, but unlike magical damage, it goes through magic immunity. Currently there are only three spells with this damage type. Those spells are Tinker's March of the Machines, Enigma's Midnight Pulse and Doom's Doom.