Everything in the world of the Clow Cards was fine until a mysterious boy named Eriol arrived and turned Tomoeda upside down with magical (and emotional!) turmoil! Sakura soon finds that she has the ability to create a more powerful form of Clow Card, a Sakura Card as Tomoyo calls it, which is a card totally concentrated under the Clow Mistress, Sakura's, powers. Eriol is there as a reincarnation of the Clow, he adores Sakura and Syaoran as the Card Captors, he even refers to Syaoran as his "cute little descendant" Whaddya think Syaoran thought of that and constantly does nice things for Sakura, which really sets off Syaoran, totally insane with jealousy. Sakura and Syaoran grow a lot closer thoughout this season, from Sakura falling asleep in Syaoran's arms to Syaoran following her to make sure she stays safe from danger. The final battle against Eriol, to save the world from eternal sleep, even marks Syaoran's confession. The end of the season is Syaoran's return to Hong Kong, and Sakura's emotional struggle to figure out her feelings. In the end Syaoran gives Sakura his bear (Tomoyo's little legend about two people staying together forever if they gave one another teddy bears with their name sown in them. I think Tomoyo-san has been hanging around Yamazaki too much and leaving the end open for the the second movie.Sakura Cards were originally Clow Cards. But in the Sakura Card series, Sakura uses her wand to transform each Clow Card into a Sakura Card. 

In doing so, she has more control over them as they are made with her own magical abilities. They are much stronger than Clow Cards. Sakura uses her own energy to change the cards, but in doing this, it drains her and she becomes weak and sometimes faints from the lack of energy. 

These cards are kept in the Sakura Book, which is formerly known as the Clow Book. It changes to a shade of pink just as the cards do..