Clow possessed a great deal of magical power. Using his power, he created the Clow Cards, which are a unique blend of Eastern and Western magic. His father was English and his mother was from the East so he was familiar with both cultures.
Clow Read created the Clow Cards first. The Clow Cards are individual spells but they are meant to be free spirits as well. They began to wreak havoc and become uncontrollable so Clow created the Clow Book to keep their powers from spilling over. Then he created Cerberos and Yue to guard the Clow Book and Clow Cards from anyone who might abuse their powers.
Cerberos is the Sun Guardian, drawing his powers directly from the "Firey" and "Earthy" cards. Yue is the Moon Guardian and drew his powers from the moon and Clow's magic and cards. Yue loved his master in a romantic sense but Clow Read didn't reciprocate the feelings. He treated and loved Yue as a son since he created both of them himself.
When Clow died (his own choice since he lived for over 200 years using magic), Yue was saddened and this caused him not to want anyone else to be his master. So Clow gave Cerberos the privilege of choosing a candidate to become the new Clow Master but he chose Yue to be "The Judge" who will determine if the candidate was worthy enough of becoming the Clow Master of the Cards and Book. Clow Read sealed the Clow Cards within the book, then the two Guardians, and finally sealed it with the Clow Sealing Wand. Then Clow Read died. The Clow Book was lost until it found it's way into Kinomoto Fujitaka's library where it awaited its new master. Kinomoto Sakura.