Reanimator decks put expensive creatures into the graveyard and return them to the battlefield for considerably less mana than casting them normally. Discard and self-mill let the graveyard function as an extension of the hand.
Exhume and Dance of the Dead provide additional reanimation effects, while targets such as Atraxa, Grand Unifier or Archon of Cruelty deliver immediate value. The archetype trades normal mana progression for graveyard setup.