All moves used by or against the ability holder cannot miss. It also allows the ability holder or its foes 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 ability holder and any Pokémon targeting it.
In Diamond and Pearl, there was a glitch in which Pokémon that used Protect or Detect could still get hit 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 the ability holder is leading the party, the wild Pokémon encounter rate increases by 50%.