Clow Reed was the creator of the Clow Cards. He made them in order to make tasks easier for him. He also created the book, Kero, and Yue to protect the cards. Clow had EMENSE magical powers. After he died, the cards were somewhat lost. Fujitaka found them, couldn't open the book, and kept them in his study. When Sakura found them and they escaped, she became the cardcaptor. She often senses Clow Reed's power when strange things happen. That's how she knows a card is near. Syaoran is a decendant of Clow. Eriol is part of Clow's incarnation, so he too has emense power.