Damage Types
Thirteen types: acid, bludgeoning, cold, fire, force, lightning, necrotic, piercing, poison, psychic, radiant, slashing, thunder. Resistance halves damage of a type; vulnerability doubles it; immunity negates it. Apply in that order against a single instance of damage.
Dropping to 0 Hit Points
At 0 HP you fall unconscious (unless the damage killed you outright — see massive damage). Healing that raises you above 0 ends the unconsciousness. Any healing works — even 1 hit point brings you back, though you stay prone until you use movement to stand.
Death Saving Throws
On each of your turns at 0 HP, roll a d20 (no modifiers): 10+ is a success, below 10 is a failure. Three successes: you stabilize (unconscious but no more rolls). Three failures: you die. Rolling a 1 counts as two failures; a natural 20 restores 1 hit point. Taking damage while at 0 HP counts as one failed save — a critical hit counts as two.
Massive Damage & Stabilizing
If damage reduces you to 0 and the leftover damage equals or exceeds your hit point maximum, you die instantly — no death saves. A stable creature (or one healed to 1+ HP) stops making death saves; a stable creature regains 1 HP after 1d4 hours. An ally can also use an action to make a DC 10 Wisdom (Medicine) check to stabilize you.