I learned about xxxHolic the same way a lot of people probably learn about a series they like: a friend told me I should read it.
xxxHolic is a supernatural series set in Tokyo (and occasionally other locales of Japan) and produced by CLAMP, a four-woman writing/drawing group. It's the story of Watanuki, a teenage boy who can see spirits and who is constantly chased by them. To get rid of his power (and to save his life), he makes a deal with Yûko, also known as the Time-Space Witch and who has the ability to grant wishes, to work for her in exchange for taking his ability away. However, Yûko's powers are based on equivalent exchange: because removing Watanuki's ability to see spirits and also his unconscious, unwanted ability to draw them to him is such a difficult task, he has to work for her for a very, very long time before it's paid off. (If it sounds a little bit like the philosophy of alchemy in Fullmetal Alchemist, that's because it is. If you don't know what Fullmetal Alchemist is, don't worry, I'll get to it.) So the two of them get off to a rocky start.
At first the series focuses only on Watanuki and Yûko and the fates of various customers who come into Yûko's shop looking to have their wishes granted, but as it goes on more characters appear: Himiwari, a girl at Watanuki's school who he has a crush on, and who always has strange things happen around her; and Doumeki, another classmate and the son of a Buddhist temple priest, who also has unusual powers and who Watanuki takes an instinctive dislike to. (Actually, it's more like irrational hatred.) Slowly these two become more and more involved in the wishes being granted in Yûko's shop, and in the lives of Watanuki and Yûko themselves.
Compared to other manga series, xxxHolic is heavily based on Japanese customs and superstitions. The translator maintains honorifics (ending words in Japanese that are attached to names, which serve the same purpose as English "Mr." and "Mrs.") throughout the dialog, and some words that would be hard to translate into English are left in their original Japanese. The most common one of these is "hitsuzen," a difficult word and an even more difficult concept (not even Watanuki knows what it means when Yûko first says it!): boiled down, it's the opposite of coincidence, the belief that something happens for a reason and there was no other possible way that things could have occurred--a theme throughout the series that some characters accept, and some fight tooth and nail. Fortunately, each volume of the series has notes on honorifics and jokes or puns that didn't quite translate right from the Japanese, so you won't get lost reading.
xxxHolic is a change from CLAMP's previous work in two ways: the art style is different, because the previous artist took a background role due to wrist problems; and the series is intertwined with another manga that CLAMP was producing at the same time, called Tsubasa Reservoir Chronicles. The early volumes of the manga rely heavily on reading Tsubasa Reservoir Chronicles at the same time to get a full idea of what is going on; but over time the series settles into its own world and the characters' friendships -- and antagonisms -- change and grow.