This Synopsis was written by me.
Sakura Kinomoto was just an average fourth grader until the one fateful day when she discovered an old book in her dad's study. Promptly after she opened the seal on the book, magical cards flew out of it and escaped into town. The book had been made by a sorcerer named Clow Reed, who had placed those cards, called Clow Cards, into the book in order to seal them away and prevent them from causing trouble. After Sakura had broken the seal, Kerberos, the guardian beast of the Clow Cards, emerged from its cover and entrusted Sakura with the task of finding and collecting all 52 of the magical Clow Cards. He explained that each card is alive and possesses great magical potential, and that normal forces are no match for them - that magic can only be conquered by magic. And that if the cards aren't collected, "a great evil will befall the world." Since Sakura was able to break the seal on the Clow Book, it showed that she did have some magical powers. Using the magical sealing staff to return the Clow Cards to their card forms, as well as help from her best friend Tomoyo and Kerberos (nicknamed "Kero-chan" because he was dwelling in his smaller, cuter form at the time)to help her, Sakura set out to collect the cards.
Sakura's adventures reach a turning point when Shaoran Li, a decedent of Clow Reed, also wants to collect the Clow Cards. At first, he appears to be Sakura's rival, but as time goes on, he and Sakura become great friends. After Sakura collected the Clow Cards and passed the Final Trial to see if she was worthy of them, she had to turn them into Sakura Cards so that they'd draw their magical powers from her instead of from Clow Reed. This later part of the series also introduces the mysterious Eriol, who's a new student at Sakura's school, but is not all that he appears to be. Even though all the cards had been captured, strange occurrences were still happening throughout Tomeda, and Sakura was constantly sensing Clow Reed's presence! But he died a long time ago, didn't he? Sakura's going to have to change all the Clow Cards fast or else an "evil" may soon very well befall the world!