The first six volumes of manga (the first and second seasons of anime) cover the Clow Card Arc, in which Sakura sets out to recapture the Clow Cards and become their master.
The cards (19 original cards in the manga, 53 cards in the anime) each represent a force (i.e. the Windy Card) or concept (i.e. the Maze Card).
The new Sakura Cards have their own unique appearance, different from the Clow Cards.