Minecraft wiki entry for Armor, associated text: Direct (melee) attacks from mobs and players.
This includes the Strength and the Weakness effects and damage-increasing enchantments.
Getting hit with an arrow, a shulker bullet or a thrown trident.
This includes extra damage from enchantments (except shulker bullets).
Getting hit with a fireball from a ghast or blaze, or a fire charge.
Laser beam of a guardian or an elder guardian.[verify]
Damage from Thorns enchantment, or spikes of guardians and elder guardians.
Touching a damaging block (fire, lava, magma block, cactus or sweet berry bush).
Touching a pufferfish.
Explosions.
Fireworks with at least a firework star in the recipe.
Lightning.
Getting hit with a falling anvil.
Getting hit with a snowball from a player‌[upcoming: JE Combat Tests] or snow golem.[more information needed]
Getting hit with an egg from a player‌[upcoming: JE Combat Tests][more information needed]