Well as far as I know, Clow Reed invented them, and they can tell the future. Clow Reed was a very powerful magician, too powerful in some aspects. His mother was Chinese and his father was English. He used eastern and western magic. He also created Yue and Cerebros. They are both the seals of the Clow Card Book. When he created this it is said he split into two people. One of them is Eriol and the other person isn't really known, In all reality its either Sakura's Dad or Toya. There is not too much else to know about him. There are 52 Clow Cards and they are guarded by the Seal, Cerebros. Some cards are good, some are bad. Some cards are really smart, some cards are REALLY dumb. Most of them just want to play and have fun. The cards have different figures on them, some of them elements Some of them objects Some of them Actions. I've noticed that most of the cards have a human like figure on them, and all of them are female. I guess this is because that's what CLAMP draws the best. Anyhow, with these cards, they do certain things like The Wood can make leaves or trees grow, The Sleep can put people to sleep in an instant. So Sakura gets a certain amount of cards, and she can use them to capture other cards, or help her to do so. Sakura, along with help from Sayoren will catch all the Clow Cards before they wreak havoc on the Earth. (These Cards Are Like "Magic" Cards)