Clow Reed is the maker of the Clow Cards, the Clow Book, Cerberus, and Yue. He's THE original Card Master. He had premonitions about the future. He's intelligent. He's nice. He's wise. He's powerful. He's a bishie. ... Man, Clow Reed is one heck of a sorcerer.
Not much was ever known about Clow Reed. I mean, even though there IS one episode dedicated to him (Episode 68, Sakura and Clow Reed's Past), little is still revealed. OK, so what DO we know about him?
Clow died WAY before the CCS storyline. The Li Clan is his descendants...The Li Clan knows magic because of that fact. And we all know that Li Syaoran is Clow's direct descendant. And...somehow...Sakura got powers... Hoe? How the heck did she fit in in all of this?! (Scroll down to read the spoilers.)
Actually, episode 68 doesn't give very much information about the life of Clow. I think it has to do more on his personality. For some reason, he likes to speak in a not-so-direct way. He leaves it to his listeners what he actually means. We also see that Yue and Cerberus are very attatched to their original master. They've had lots of great times together (Cerberus even told Sakura some, partucularly the funny Bubbles incident, in episode 58), and Clow treasures them a whole lot...Just like Sakura does. Clow is always smiling, much like Yamazaki-kun... (Wait a second...You don't think...Yamazaki is...*grin* Never mind. Just exploring the possibility:) And, he has complete trust in Sakura. Also, he said, by far, my favorite quote of all time: "We cannot live in the past forever. The future is always ahead of us."
Clow, as I've said, had premonitions regarding the future. To prepare for that, he had appointed Cerberus to choose the new Card Master, and Yue to test and verify it. He had written books and books about stuff...
Through time, Clow gets to be reincarnated...in the CCS storyline, he has two incarnations. One would be Hiiragizawa Eriol. He was the first one to regain his memories as Clow. According to age AND chronologically. He gets into action way before the other incarnation even realizes his past life as Clow...and that incarnation is Kinomoto Fujitaka. Hm. I guess THAT would explain why Sakura has powers. She's the half-daughter of Clow!