'I am afraid that the nag - I'm sorry, the centaur - knows nothing of cartomancy."
-- Sybill Trelawney
A form of Divination where playing cards are used to tell the future.
Trelawney uses cartomancy in HBP10, wandering the corridors predicting doom with an ordinary deck of playing cards:
"Two of spades: conflict," she murmured, as she passed the place where Harry crouched, hidden. "Seven of spades: an ill omen. Ten of spades: violence. Knave of spades: a dark young man, possibly troubled, one who dislikes the questioner —"
Later in HBP25, she pulls out a card and refers to it as the Lightning-Struck Tower. This doesn't refer to a card in the normal playing card deck but instead refers to a card in the sixteenth Major Arcana card in the tarot. This card is usually called simply The Tower, but the image on one of the most famous tarot decks, the Rider-Waite deck, portrays a tower being struck by lightning.