We currently have these Keywords in the game:

Armor: Prevents the first point of damage from each individual damage source.

Banner: Provides +1 Power to your Hero.

Guard: Units with guard prevent your Hero from being attacked.

Lifesteal: Damage dealt to the enemy hero by Units or Spells with Lifesteal gives your hero that much health.

Stealth: Units with Stealth cannot be attacked unless their Hero has been damaged this turn.

Wither: Damage dealt to Units (but not heroes) by Units or Spells with Wither will reduce both their health, and their Power.

More on Keywords in this blog post.