What do each of the Tarot cards do?

Solution 1:

The Binding of Isaac wiki's got a pretty comprehensive (if slightly generalized, and minorly inaccurate) list. Remember that all cards are a single use item, so once they are used they vanish.

• O The Fool: Teleports the player to the starting room for that floor.
• I The Magician: Gives you the use of Spoon Bender (homing shots) until you leave the current room.
• II The High Priestess: Summon's Mom's foot stomp on a random enemy. Can also hit the player, and will do so if there are no enemies in the room.
• III The Empress: Gives you the use of Whore of Babylon until you leave the room. Whore of Babylon is Eve's curse, but unlike her version, this just hurts you half a heart then increases your damage.
• IV The Emperor: Teleports the player to the Boss Room.
• V The Hierophant: Spawns two soul hearts.
• VI The Lovers: Spawns two hearts.
• VII The Chariot: Acts as a single-shot use of My Little Unicorn.
• VIII Justice: Spawns bombs, hearts, coins, and keys around the player.
• IX The Hermit: Teleports the player to the Mini-boss Room, or the shop if a mini-boss room is not available.
• X Wheel of Fortune: Spawns a Slot Machine.
• XI Strength: Makes the player bigger, increasing maximum health and damage until the player leaves the room.
• XII The Hanged Man: Gives you the use of Transcendence (flight over gaps, rocks and spikes) until you leave the current room.
• XIII Death: Gives you a single use of The Necronomicon (massive damage to all enemies in the room).
• XIV The Tower: Gives you a single use of Anarchist's Cookbook (spawns multiple bombs around the room at random).
• XV The Devil: Gives you a single use of Book of Belial (increased tear damage) until you leave the current room.
• XVI Temperance: Spawns a Blood Donation Machine.
• XVII The Stars: Teleports the player to the Item Room, or acts as Telepills if there is no item room (i.e. in The Womb and Sheol).
• XVIII The Moon: Teleports the player to the Secret Room.
• XIX The Sun: Completely heals the player, does damage to all enemies on screen and acts as a Treasure Map and Compass for the current level only.
• XX Judgement: Spawns a beggar. If you have no money, spawns 1 heart, coin, bomb and key instead.
• XXI The World: Acts as a Treasure Map and a Compass for the current level only.

Tarot Cards II, III, XVII, and XIX were added in a patch on November 1 2011.