Damage Protection

The relevant wiki articles are Defense and Damage.

Each monster has one or more attacks. Each attack has a certain mix of damage and status. For example, a Tier-3 fire zombie has these four kinds of attack:
* swipe: normal, shadow, fire
* lunge: normal, shadow
* bite: normal, shadow
* breathe: fire

Meanwhile, your armor protects you against a certain mix of damage and status. For example, Dread Skelly protects you against normal, shadow, poison, and freeze, while Vog Cub protects you against normal, elemental, and fire.

Suppose that a fire zombie swipes you. Here is what happens to you, depending on your armor:

* Dread Skelly: The armor absorbs some of the normal damage and some of the shadow damage. The rest of the damage hits you --- meaning that it gets subtracted from your health. This armor also lets all of the fire through, so you have a high chance of catching fire for a while.

* Vog Cub: The armor absorbs some of the normal damage, but none of the shadow damage. The elemental protection is not relevant, because the attack does not contain any elemental damage. So you end up with less health than when you were wearing Dread Skelly. This armor also resists fire somewhat, so you have a lower chance for less severe fire.

Is that what you were asking?

(Disclaimer: I'm not an expert on how status works, so don't take that part too seriously, but you get the vague idea.)

You would take less damage from that undead monster.

If you're using an armor piece that defends against the damage type a monster which attacks you deals, you'll take less damage from the attack.

Remember that shields don't affect your stats in any way at all unless you're actually blocking with it (except for a few specific ones at the bottom of this post). Say you have elemental resistant armor, but a shadow resistant shield. You won't take less damage from a fiend or undead, but your shield will. Your shield also won't take reduced damage from a construct or gremlin, but you will.

Shield health bars don't affect your HP - You'll have the same HP when you're using a proto shield as you will when using an Omega Shield.
The only shields which affect your stats are the Scarlet Shield, Swiftstrike Buckler, and Bristling Buckler line.

the Torto shields also decrease your movement speed a bit.

Thank you, Bopp, your explanation was just what I was looking for.

You're welcome. Happy travels.