The Clow book

'When the seal on them is broken, a catastrophe will befall this world...' 

General 
The Clow book is the magical book Sakura (see page Card Captor Sakura) finds after several weird happenings throughout the day. When she finally comes home after a school day she hears weird noises in the basement. As finding something shining in the dark, she picks up the book from the self and that is where the adventure startes. In the book are 52 cards to be found with all their own name and power.

The Begin
As said here above, in episode 1 of Cardcaptor sakura or in the first chapter of the manga, she finds The Clow book in the basement on a shelf after having the whole day through weird things happen to her.
She picks the book up from it's shelf. She opens the book and in there she finds weird shaped cards inside it. On the first card it says 'The Windy' and so after calling in out loud the cards get tornado of wind and fly off to different places. After that Kero or Cerberus (The beast guardian) comes out of the book which is the start of an new adventure to gather the clow cards!

Clow
The Clow book and cards are made by sorcerer named Clow Reed. He is also the master of Cerberus and Yue. Bron from an English father and Chinese mother, he created an magic with both Eastern and Western elements.