Plot: Kimihiro Watanuki is a high school student plagued by yōkai, spirits with a strong attraction to him. The spirits are invisible to others and encounters with them are extremely troublesome. When he stumbles into a shop that grants wishes, however, events in his life promise to become more unusual.
The shop is owned by Yūko Ichihara, a mysterious and beautiful (sometimes playful) witch of many names and esoteric renown. For a price, she offers to grant Watanuki's wish to be rid of the spirits. The price, according to Yūko, must be of equal value and so, as payment, he temporarily becomes Yūko's part-time employee.
Watanuki's job consists of small errands dealing with the supernatural and household chores. His love interest, Himawari Kunogi, and "rival," Shizuka Dōmeki, occasionally join him in his work as per Yūko's request. The three become increasingly close despite Watanuki's annoyance with Dōmeki.
The world of xxxHolic is directly connected to several other of Clamp's works through dimensional traveling. The series is directly connected to Tsubasa: Reservoir Chronicle, while other series including Cardcaptor Sakura, Legal Drug, and Magic Knight Rayearth are referenced at different times.
Yūko constantly refers to hitsuzen (必然 ), meaning inevitability, destiny, or fate, as being the driving force in the world. She states that there is no such thing as coincidence, and that every single action and decision is inevitable, because without them, other related events in the future could not happen.
(source: Wikipedia)

Related anime: Tsubasa Chronicle, Card Captor Sakura, Magic Knight Rayearth