Card Captors began as a Japanese  Manga comic, Created by a bunch of people named, Clamp. It first appeared in a manga magazine, Nakayoshi! In April 1998, it was made  into an anime.  Then they created a full-length film released in Japan. 

Sakura was a 4'th grader, who lived her  life in the town of Tomoeda, Japan. One day she heard a strange noise coming from her father's library  in the basement. She went down to the basement and found a book called "The Clow" which had a big golden lion on the front. Inside was a card called "The Windy" as Sakura said "windy", a wind was released, which blew all the cards out of the book. The cards caused trouble everywhere they went.

The book was actually the holder of the magical Clow Cards, created by Clow Reed. Just then a little bear creature with small wings named Kero, who is really Cerberus, the lion on the front of the book. He said he had dozed off while watching over the cards, he then appointed Sakura, card captor, to put the cards back in the right place.  He gave her a key, which turns into a magical Sealing staff when ever Sakura says "Release", so she could capture the cards and put them back into the book, before they Create disaster throughout the world.