Graveyard is a spell that keeps spawning skeletons in an area on your opponent's side for a while. Cast on the tower, the skeletons deal constant damage, making it the win condition of control decks.
Cast Graveyard on the enemy tower; while the skeletons attack, stop the defense with Poison or Freeze. Sending it with a tank (Knight, Giant) makes it easier for the skeletons to reach the tower.
Tips
Casting Graveyard closer to the back of the tower helps the skeletons reach it more safely.
Cast it when your opponent's splash cards are out of cycle.