The Joker is an NPC like Satan in the Devil Room, when you enter a Joker Room you are presented with 2 choices, Trick or Treat.

Trick has a 4/6 chance to do something bad and a 2/6 chance to do something good.

Treat has a 5/6 chance to do something good and a 1/6 chance to do something bad.

So why choose Trick? Because you may fight Joker who holds unique and powerful items.

Possible Tricks:
Trick: Ambush- Spawns random enemies to fight you, they drop a key, bomb, coin or heart at the end.
Trick: Boss- Spawns a random boss to fight you, they drop their normal boss loot and have normal boss health.
Trick: Showdown- Teleports the player to the boss room.
Trick: Joker- Joker fights you and when beaten will drop a unique item.
Trick: Devil- Spawns a devil room.
Trick: Treat- Spawns a random item (not a pickup).

Possible Treats:
Treat: Pickup- Spawns random pickups (Coin, heart, bomb or key).
Treat: Prize- Spawns a random item (not a pickup).
Treat: Health- Restores to full health.
Treat: Stats- Increases a random stat.
Treat: Secret- Teleports the player to the secret room.
Treat: Trick- Lowers the players health by 1 (removes a heart container not damages hearts).