A power weapon (typically a sword or axe, but more rarely a glaive, halberd or mace) is sheathed in the lethal haze of a disruptive energy field. When a blow is struck, a crackling blast of energy tears at the target, eating through armour, flesh and bone with ease.
A model armed with a power weapon ignores Armour Saves. Note that some power weapons may provide the attacker with bonus Strength in addition or wound on a fixed dice score regardless of the enemy's Toughness characteristic. These will be detailed in the appropriate Codex.