The three defences

Armour pieces (helmets, body armour, legs and boots) come in three types and in hybrids:

Type	What it does	Best for
Armour	Cuts physical damage from hits, weaker against big hits	Knights
Evasion	A chance to avoid melee and physical ranged attacks entirely	Paladins and monks
Energy shield	A second life bar above your life	Sorcerers and druids

Hybrids. There are armour and evasion, armour and energy shield, and evasion and energy shield hybrids, giving half of each value. They appear at seven of the twelve tiers: 3, 5, 7, 9, 10, 11 and 12.

Shields are block only: a shield's number is its chance to block. Spellbooks add some base energy shield.

Same strength, different behaviour. At the same tier, a full set of any type is about equally strong against a same-level monster, but each reacts differently:

Armour helps against many small physical hits and less against boss slams.
Evasion avoids hits completely, including their poison, burning or bleeding side effects, but does not help against spells.
Energy shield covers every damage type except poison, and recharges when you are not hit.

Where you find them. The type shows in the item name and tooltip: the sprite look matches the type, so knight plate is armour, leather is evasion, robes and hats are energy shield. See Armour and block, Evasion and Energy shield.

Affixes add to the defence you use: "increased armour", "increased evasion" and "increased energy shield". Resistances and block work with any type (Damage types and resistances).

On the tree. The passive tree puts evasion near the Monk and Paladin, energy shield near the Druid and Sorcerer, armour near the Knight, and hybrid nodes at the borders (The passive tree).

Flasks. The Granite flask raises armour (Flask types and tiers).