The Clow Cards are basically cards that were stored in a book called the Clow that happened to be in Sakura's father's library. Sakura opened this book, and read the name of the card "Windy", causing the cards to all blow out of the book, while the keeper of the cards, Kerberos or Cerberus, (also known as Kero) was sleeping on the job. The cards all represent and have different elements or powers. Sakura, assigned the job of Card Captor, has to go and collect all the cards so that they will not cause havoc.

This is only a brief description right now. I will add more later.