No Guard makes all moves used by or against a Pokémon with this Ability cannot miss. It also allows Pokémon to be hit during the charging turn of Fly, Bounce, Dive, Dig, Shadow Force, Sky Drop and Phantom Force. In battles with multiple opponents, the effect only applies to the Pokémon with No Guard and any Pokémon targeting it.
In Diamond and Pearl, there was a glitch in which Pokémon that used Protect or Detect could still get but only if the move had less than 100% accuracy with less accurate moves being more likely to break through the Protect. Moves that always hit were blocked as well as one-hit KO moves. This effect was fixed in Platinum and the games that followed.
Outside of Battle
Outside of battle, if a Pokémon with No Guard is leading the party, the wild Pokémon encounter rate increases by 50%.