Cartomancy

Cartomancy was the art of reading cards to gain insight into future events. There was a special kind designed especially for this purpose, known as a tarot deck, but standard playing cards could also be used.

History
In 1993, when Harry Potter, Ron Weasley, and Hermione Granger arrived at the Divination classroom for their first-ever lesson, the shelves lining the walls were stuffed with packs of worn playing cards and other items.

In 1996, Harry Potter was going down a corridor when Sybill Trelawney came around the corner, shuffling through and reading a deck of playing cards as she walked. Harry ducked behind a statue to avoid being seen, but when Trelawney came to the knave of spades — which she said indicated "a dark young man, possibly troubled, one who dislikes the questioner" while speaking to herself — she stopped right in front of it. However, she quickly convinced herself this reading must be incorrect, and continued on her way.

In 1997, on the night Albus Dumbledore was to die, Harry again encountered Trelawney in a corridor. She told him that she had done several tarot readings, drawing the lightning-struck tower every time, an extremely ill omen. She also remarked that Firenze, who she had consulted about this portent, knew nothing of cartomancy.

Card meanings
Playing cards, according to Sybill Trelawney:
 * Two of spades — "conflict"
 * Seven of spades — "an ill omen"
 * Ten of spades — "violence"
 * Knave of spades — "a dark young man, possibly troubled, one who dislikes the questioner"

Tarot cards, according to Sybill Trelawney:
 * The lighting-struck tower — "calamity" and "disaster"