First of all, the story of Cardcaptors is about a fourth-grade girl named Sakura Avalon, who stumbles apon a "clow book", a book which contains many "clow cards", cards which have strong magical powers. Sakura accidentally releases all of the cards, not knowing what they are, but later learns that she is destined to capture them all again and be their ruler, the keeper of the clow cards, the clow mistress. She has to capture the cards to prevent them from creating havoc all over the world.

Sakura is guided by a sweet, video-game loving little creature named Kero, who emerges from the clow book after Sakura releases all the cards. He resembles a stuffed animal, but as Sakura captures more cards, and finally, all of them, Kero regains his magical powers and takes on his true form of the impressive Keroberos.

Sakura's best friend Madison, who loves fashion designing and video cameras, becomes Sakura's faithful sidekick, aiding on cardcaptures, designing wonderful costumes for her friend to wear, and taping as many cardcaptures as she can.

Sakura also has a rival in Li Showron, a descendant of Clow Reed, the magician who created the clow cards. Li seems to think that Sakura is clueless, and that he deserves to be the only Cardcaptor. But over time, the two start helping each other out, and even fall in love, but that's another story. ^^
Li's friend Meilin, who seems to be his girlfriend, is also a trial to Sakura, competing with her at every chance, sticking up for Li even when he doesn't want it. ^^ But over time, even those two begin to understand each other, and become friends.