When Nature Power is used it turns into a different move depending on the current battle terrain, as listed below.
Although it's a Normal type move, the user only receives STAB if the move it turns into is their type. For example, Sawsbuck (Normal/Grass) would not obtain STAB if Nature Power turned into Hydro Pump, but Ludicolo would.
In double/triple battles, due to the different moves having varying targets Nature Power is set to target the user. However, moves that target multiple Pokémon such as Rock Slide or Earthquake still hit multiple foes and/or teammates as normal. Moves that target a single foe will strike the Pokémon directly opposite the user, or an adjacent foe if there is no Pokémon opposite.