What reduces or avoids the damage you take — and how it gets bypassed.
Armour
Armour reduces Damage taken from Hits. By default, Armour only applies to PhysicalDamage.
Damage reduction from Armour is proportional to the amount of Damage, and is more effective at reducing smaller hits.
Evasion
Evasion Rating grants a chance to Evade enemy Hits, preventing them from Hitting you at all. Exact chance to Evade also depends on the attacker's Accuracy.
Energy Shield
Energy Shield protects your Life by taking damage instead. Rapidly Recharges if you don't lose Energy Shield for a short time.
Chaos damage removes twice as much Energy Shield.
Damage from Bleeding and Poison bypasses Energy Shield to remove Life directly.
Runic Ward
Runic Ward is a last line of protection, absorbing fatal damage instead of your Life. If you take damage that would cause your Life to reach 0 while you have Runic Ward, you will drop to 1 Life and your Runic Ward will take the remaining damage. You will still die if you do not have enough Runic Ward to absorb the remaining damage. Runic Ward does not protect against Life loss that is not caused by taking damage.
Runic Ward constantly regenerates at a default rate of 5% per second.
Monsters can also have Runic Ward. While a monster has Runic Ward, it cannot be Culled.
Block
Blocking completely prevents the damage of an incoming Hit.
You will still take any Stun from the Blocked hit. You can't Block while Stunned or Frozen.
Some Skills used by Bosses cannot be blocked. These are indicated by a red glow and audio cue during the windup of the Skill.
Chance to Block
Chance to Block only includes modifiers that are explicitly chance-based. Guaranteed Block (such as raising your Shield) does not affect your Chance to Block.
Resistances
Resistances reduce damage taken of the corresponding damage type — Fire, Cold, Lightning or Chaos — up to a Maximum. Fire, Cold and Lightning Resistances are Elemental Resistances. Resistances can be improved with Equipment, Passives & Quest items.
Your Elemental Resistances are lowered as you progress through the game. Elemental Resistances are a vital defensive tool, and should be one of the first things to check if you're having difficulty surviving.
Maximum Resistances
The default maximum for Elemental or Chaos resistances is 75%. Maximum resistances cannot be raised above 90%.
Armour Break
Some Skills, Items, Support Gems and other effects can Break Armour, which lowers a target's Armour by a specified amount. If this brings the target's Armour value to 0, their Armour is Fully Broken for 12 seconds, or 4 seconds for players. On top of not benefitting from Armour, non-player targets with Fully Broken Armour take 20% increased Physical damage from Hits.
Players Break 3 times Armour against Normal Monsters and 2 times Armour against Magic Monsters.
Resistance Penetration
Penetration causes the target's corresponding Resistance to be treated as lower than its actual value by the specified amount when of calculating Damage taken from your Hits. Resistances can only be Penetrated down to a minimum of 0% by default.
Since Penetration only affects Hits and applies to the target's defensive stats rather than your own offensive stats, it does not affect damage with Ailments.
Ignoring Resistances
Ignoring Resistances means your damage cannot be modified in any way by the target's Resistance stats.
Exposure
Exposure is a type of Debuff that lowers the affected enemy's TotalElemental Resistances. By default it lowers Resistances by -20% and lasts for 4 seconds, though some sources of exposure can override these values. Like most sources of lowering Resistances, this can cause the enemy's Resistance to become negative.
Higher Rarity enemies are less affected by Exposure:
15% less Exposure effect on Magic monsters
30% less Exposure effect on Rare monsters
50% less Exposure effect on Unique monsters
Overwhelm
Overwhelm negates a certain amount of the target's Physical damage reduction, but never more than the total Physical damage reduction the target has.