In The Binding of Isaac, some of the items that you can pick up are special numbered Tarot cards with different special effects when you use them. Some of these trigger events that I don't want to happen; I used The Emperor under the impression that it would save me from peril but it took me straight to the boss of the floor. So in order to avoid these kinds of situations and use each card appropriately, I'd like to know: what does each card do?
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.