What each stat actually does

Most stats do what they say. The ones worth reading twice are the capped ones, and the leeches - both can be completely inert without looking like it.

Offence

attack power
Increases the damage of your attacks. Joins the shared pool with every other increase to attack damage.

spell power
Increases the damage of your spells. Most card and item damage counts as spell damage, so this reaches further than it looks.

attack speed
Makes you swing more often. It raises everything that triggers per swing, not just your weapon damage - on-hit cards, Nth-hit counters and leech all speed up with it.

critical hit chance
How often your damage critically hits. Works on attacks and spells alike.

critical hit damage
How much extra damage a critical hit deals. Worth nothing on damage over time, which never crits - a bleed or burn build gains far less from this than a hit-based one.

Multi Hit
The number of extra targets your swing hits. The extras deal 80% damage and trigger your on-hit effects at 80%, and they share the swing's critical hit roll rather than rolling their own.

Defence

damage reduction
Reduces the damage you take. Capped at 80% - stacking past that does nothing at all, so check your sheet before chasing more.

armor
Reduces physical damage taken. It does nothing against fire, ice, energy, earth, holy or death.

max health
Your health pool. Several effects convert it into damage rather than defence, so it is not purely a survival stat.

max mana
Your mana pool. Some barriers scale off it rather than off your health.

Sustain

healing power
Increases the healing you do.

healing received
Increases healing done to you, from any source - your own heals, an ally's, and tonics.

life leech chance
How often an attack restores health to you. It only rolls while you are below full health, so at full health the stat is completely inert. Damage over time never leeches.

life leech amount
How much health an attack restores. A flat value, not a share of the damage - hitting harder does not leech harder.

mana leech chance
How often an attack restores mana to you. Same rule as life leech - it only rolls while you are below full mana.

mana leech amount
How much mana an attack restores. A flat value, not a share of the damage.

Utility

cooldown reduction
Shortens your spell cooldowns. Capped at 80% - anything above that is wasted.

movement speed
How fast you walk.