Deadly Strike/Critical Strike
Deadly Strike (DS)/Critical Strike (CS) are both a chance to double your Physical damage.
They apply to both Melee and Ranged attacks, but not to Spells.
They do not apply to Impale, Dragon Talon, Dragon Tail, Dragon Flight, Sacrifice, or Smite.
They do work with the Blade Traps.
Skills that convert Physical damage to a different type (Magic, Fire and Cold Arrows, Lightning Bolt, Fists of Fire, Berserk, Concentrate, and Frenzy (when synergized by Berserk)) do so after DS or CS are applied, so converted damage is doubled.
They will modify your final Physical damage with Vengeance but have *no* effect on the Elemental damage caused by Vengeance.
Sources of Deadly Strike stack except from a weapon which is not being used to inflict that particular blow.
Different types of Critical Strike do *not* stack (e.g. Barbarian Weapon Masteries and Amazon Critical Strike).
There is no point to having more than a 100% chance of DS/CS.
Critical Strike and Deadly Strike have the same mechanics and only one of them can trigger on any given attack (so a player can't get quadruple damage)