Clow Reed is the creator of the Clow Cards (also referred to as "The Clow"), a series of cards resembling tarot cards with various magical powers within them. Additionally, he is the creator of Yue and Cerberus, assigned to protect The Clow and ensure that it gets passed on properly to new masters. Syaoran Li (Li Showron) is a distant relative of Clow, as Clow's mother's family is the Li Clan.
When Clow was nearing death, he chose Sakura Kinomoto (Sakura Avalon in Cardcaptors) to be the new mistress of The Clow. Sakura was not yet born at the time, but Clow's magical ability allowed him to foresee most of the future and plan many of the situations that would enable Sakura to succeed in taking full possession of the Clow Cards. This is the reason that Sakura's sealing wand is pink and cute; he wanted to make a suitable wand for the young girl who would be mistress.
Very little of Clow's personal life is known, although Kero often references his liking for playing jokes, and his constant smile and the likes and interests described in his manga quarter-page indicate that he was a content and mischievous person. There are also some hints, especially in the anime, that Clow and his creation Yue had some sort of connection that went deeper than Master and creation. However, whether this was a mutual romantic interest or simply one-sided on Yue's part in unknown.
During the first Cardcaptor Sakura movie, it is revealed that as an extremely powerful magician, Clow had a great deal of enemies, one of which is the movie's antagonist, a madoushi (sorceress) who held a bitter grudge against Clow even after she died. She is not named in the original version and simply referred to as the "Madoushi"; the English dub opted to name her "Su Yung". The Madoushi, who worked as a fortune-teller, found that Clow's ability to accurately predict the future affected her own business and developed a deep hatred for him. She challenged her rival to frequent duels and eventually cast a spell that allowed her spirit to reside in a book after she died and draw Clow Reed to her. Once released from the book by Clow (or one possessing the Clow Cards), that person would become sealed in the book in her place. In the English dub, Su Yung was actually a student of Clow's and they had loved one another. However, her desire to become powerful resulted in spreading destruction rather than using it responsibly. The two were unable to see eye to eye concerning their belief as to how magic should be used and an intense fight ensued. When she lost and died, Clow sealed her in an alternate dimension of his own creation, where she sought to free herself and pass onto the next world after taking revenge on Clow.
Sakura unintentionally releases the Madoushi during a trip to Hong Kong and the sorceress, refusing to believe Clow is dead, attacks the young cardcaptor and endangers her friends. She eventually relents when she realizes she truly loved Clow; she accepts his death and vanishes.
Clow makes an appearance in episode 46 just after Sakura has completed the Final Judgment, thus becoming the new mistress of The Clow. Here Clow explains the coincidences he had planned for Sakura--situations that would allow her to recapture all the Cards. He also advises Sakura to take care of her new power, the power that changed the original sealing wand into one that is completely Sakura's.
In episode 68, Sakura uses The Return to visit Clow prior to his death. Clow recognizes Sakura but does not concisely answer all of her questions. Sakura is mainly concerned that she would never be as great or as powerful as Clow, but Clow reminds her that she cannot keep comparing herself to him. Sakura is her own self, and she has already been fulfilling Clow's final wish, which is to allow his creations (the Cards, Cerberus, and Yue) to live in happiness under a new master. During this visit, Clow gives little to no insight about the recent mysterious happenings, though Sakura finds that out from Eriol when she returns to the present.
According to Kero in the manga, Clow Reed, having both an English and a Chinese parent, was able to combine occidental and oriental magic to create an entirely new type of magic--the Clow Cards. Like his later successor, Sakura, Clow's magic is a balance between the sun and the moon. His power is of darkness, an affiliation which his reincarnation, Eriol Hiiragizawa, shares, and which manifests itself in the chant used to release the sealing staff (which uses Clow's power until Sakura transforms it under her own power of the stars).
Clow Reed's present-day reincarnation is Eriol Hiiragizawa (Eli Moon), appearing in season three of the anime. In the manga, Sakura's father, Fujitaka Kinomoto (Aiden Avalon) is also a reincarnation (but just one-half, while Eriol is the other). Eriol retains all of Clow's past memories and all of his magic. Clow was not able to split his magic apart into the two new bodies and Eriol's final wish is for Sakura to take half of his magic and give it to her father. This wish not to be the most powerful magician was probably due to Clow Reed's desire for Yuuko to stay alive, which led to the events of Tsubasa Reservoir Chronicle.
Eriol's purpose is to create situations, much like Clow did in season one, that would allow Sakura to convert the Clow Cards into Sakura Cards, putting them wholly under her magical influence and completely finishing the transition of The Clow from Clow Reed to Sakura.
Because Eriol is Clow's reincarnation, Sakura and company continuously detect Clow's presence during Eriol's trials. This leads to suspicion in Sakura when she visits Eriol's manor in episode 65 and feels Clow's presence all over, especially in Eriol's armchair. The manor was indeed inhabited by Clow when he was alive, but Sakura does not fully realize this until the end of the series.
After learning that Clow was reincarnated, Yue protests the entire situation involving Sakura having to become the new master of The Clow. Eriol, holding Clow's memories, explains that he is not the actual Clow Reed, who would never return to this world. This is why Clow entrusted everything to Sakura. However Eriol offers to talk about his memories whenever Yue wishes.
Yuuko Ichihara, a main character in xxxHolic, repeatedly talks about Clow. While the exact nature of their relationship is uncertain, they are known to have worked together to create the two Mokonas who appear in xxxHolic and Tsubasa. Fei Wong Reed, who appears in Tsubasa: RESERVoir CHRoNiCLE, is also Clow Reed's relative, and considers Yuko and Clow as barriers.
Clow Reed is mentioned in Tsubasa: RESERVoir CHRoNiCLE, and he was personally acquainted to Yūko Ichihara. The two together created the Mokonas that would help Syaoran, Sakura, Fai and Kurogane travel across dimensions. Fei Wong Reed, the mastermind who initially guides the journey of Sakura and Syaoran across the dimensions, is an ancestor of Clow Reed's; though Fei Wong refers to Clow as an interloper in his affairs. Clow Reed is also the father of the clone Sakura-hime, and hence the real "Syaoran" is his 'grandson'.
Clow had accidentally wished for Yuuko to live, thus severing her time from all dimensions. Clow regretted this wish, and no longer wanted the title of "world's most powerful magician". After "Syaoran" had reversed time, to fill in the void left by the changed destinies of those who knew of the Seal on "Sakura", Clow travelled to Clow Country, leaving behind (he 'died' and was reincarnated) those he knew and cared about (from the manga, it appears to refer to Cerberus and Yue), as well as his power (when he died, he reincarnated himself into Eriol with all his power and memory and Fujitaka, who is the father of Cardcaptor Sakura, and in turn he moved to Clow Country), and took the role of king, becoming father to the clone Sakura and Touya. He allowed the excavation of the ruins in the country under the direction of the archaeologist, Fujitaka, and also allowed Fujitaka's adopted son, Syaoran, to become a close friend of his daughter Sakura. He would later teach Yukito, the current High Priest of Clow, his knowledge of magic.
