Game Mechanics
Master guide to POE2's elemental and status effect mechanics
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PoE2's three core attributes grant only flat bonuses - they do not give the percentage scaling they did in PoE1.
Strength grants +2 Maximum Life per point (a flat bonus, with no melee or other damage). Dexterity grants +5 Accuracy Rating per point and does NOT grant Evasion. Intelligence grants +2 Mana per point and does NOT grant Energy Shield.
Spirit is a separate resource, not a scaled attribute. It pays reservations for persistent effects such as auras, heralds, permanent minions and persistent buffs. Base Spirit comes from late-campaign quest rewards, gear (sceptres in particular) and passives - it does not scale from Strength, Dexterity or Intelligence.
Life is your primary defence and scales mainly from your level, Strength (+2 per point) and passive tree life nodes.
Mana powers skills and can be reserved by persistent effects (paid via Spirit in many cases).
Energy Shield is a secondary pool that recharges automatically: base recharge is 12.5% of maximum ES per second, beginning after 4 seconds of taking no ES damage. (The old '20%/sec after 2s' framing is incorrect.)
PoE2 layers several defences. No single layer is sufficient for endgame - combine them.
Armour reduces physical damage from hits, Evasion avoids attacks entirely, Block stops hits outright, Deflection reduces hits, Energy Shield and Runic Ward act as extra pools, and recovery (life regen, leech, flasks) keeps you topped up.
Armour reduces physical hit damage by Armour / (Armour + 12 x damage taken), capped at 90%. Breakpoints: 50% needs roughly 12x the hit in armour, 75% needs 36x, and 90% needs 108x. (The constant 12 is the best-supported value.)
Evasion gives a chance to avoid attacks. Chance to be hit floors at 5%, so evade caps at 95%. Evasion uses a hidden entropy counter so you are never hit twice in immediate succession nor go unhit for too long.
Spells are not evaded and do not use accuracy - they skip the hit check entirely; accuracy and the distance penalty apply only to attacks.
Base maximum Block is 50% in 0.5.x (it was 75% at beta). Specific modifiers can raise the cap toward an absolute 90%, but some sources of higher block still let a portion of the hit through.
Deflection is a newer layer. Chance to Deflect = 150 x (1 - A / (A + 0.12 x D)), capped at 95%, where A is the attacker's Accuracy Rating and D is your Deflection Rating. A successful deflect reduces that hit's damage by 40%, and unlike Evasion it works against boss attacks.
Runic Ward (added in 0.5.0) is a secondary hit pool that activates once your Life reaches 1: incoming damage is then taken from Ward, letting you survive while it depletes. It regenerates independently of Life and does not protect against non-damage life loss. Its source is Runeforging with Verisium.
Leech in 0.5.0 has a per-hit input cap: for leech calculation only, a single hit is treated as dealing at most 40,000 damage. Hits below 40k are unaffected; a larger hit is treated as 40k for leech purposes. This caps leech input per hit, not total leeched life.