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.