Spit Up deals damage. The base power is 100 times the Stockpile counter. If the Stockpile counter is 0, the game says "But it failed to spit up a thing!". The Stockpile counter is then reset. However, the Def and SpDef increasements caused by Stockpile currently aren't.
Even though the move infobox displays Spit Up as a Status move, it deals Special damage and uses the user's SpAtk stat for damage calculation.
Even though the move infobox displays Spit Up to have a base power of 100, its base power can be as high as 300. This makes Spit Up with Stockpile counter 3 the move with the highest base power in the entire game, surpassing even Explosion and Holy Duty.