Sontra's excellent writeup above emphasizes the fairy tale components of The Lion, the Witch and the Wardrobe, but I think it is relevant to delve a bit deeper into why the book is also a Christian allegorical tale.
C.S. Lewis was an eminent author and a brilliant Christian scholar. Most of his books are serious matter, challenging the conceptions of the Christian God, and conveying Lewis' take on some of the more fundamental ethical questions in Christianity. The Lion, the Witch and the Wardrobe is a children's book, however, and Lewis uses the setting of wonderous fairy tale world to explain some central tenets of Christianity, as Lewis views them.
The heroes of the book, the four children, stumbles into another dimension. It is a world of eternal Winter and terror, ruled by a descendant of Lilith; the White Witch. There is hope, however, for it is prophesied that Aslan, the true lord of World will someday return and with his return the rule of evil will end. The children witness the return of Aslan. Christmas, personified, is the first indication of his arrival; the second is the coming of Spring. When they meet Aslan face to face, he is terrifying but good. "He is not a tame lion."
At this point of the story, one of the children, lured by greed and spite, has betrayed the others to the Witch. He has repented, but the most ancient magic in Narnia commits the life of traitors to the White Witch, and she claims him from Aslan.
"And so," continued the Witch, "that human creature is mine. His life is forfeit to me. His blood is my property."
"Come and take it then," said the Bull with the man's head in a great bellowing voice.
"Fool," said the Witch with a savage smile that was almost a snarl, "do you really think your master can rob me of my rights by mere force? He knows the Deep Magic better than that. He knows that unless I have blood as the Law says all Narnia will be overturned and perish in fire and water."
Aslan refuses to give the Witch the child, but he makes a secret agreement with the Witch. Only witnessed by two of the children, he walks to an ancient site of magic and ritual executions. There the Witch's creatures wait for him, and he allows himself to be captured, beaten and mocked. His great mane is cut off, he is tied and gagged, and the Witch kills him.
Another roar of mean laughter went up from her followers as an ogre with a pair of shears came forward and squatted down by Aslan's head. Snip-snip-snip went the shears and masses of curling gold began to fall to the ground. Then the ogre stood back and the children, watching from their hiding-place, could see the face of Aslan looking small and different without its mane. The enemies also saw the difference.
"Why he's only a great cat after all!" cried one.
"Is that what we were afraid of?" said another.
And they surged round Aslan, jeering at him, saying things like "Puss, Puss! Poor Pussy," and "How many mice have you caught today, Cat?" and "Would you like a saucer of milk, Pussums?"
"Oh, how can they?" said Lucy, tears streaming down her cheeks. "The brutes, the brutes!" for now that the first shock was over the shorn face of Aslan looked to her braver, and more beautiful, and more patient than ever."
Aslan, the great lion; the lord of the world, who explicitly could easily have avoided it or defended himself, allowed himself to be killed by the Witch, to save the life of the traitor. The children keep a wake during the night, but as dawn breaks a thunderous crack destroys the ancient magics of the site, and Aslan, alive and seemingly more radiant than ever, has returned from the dead.
Aslan and the children then finally defeats the Witch in a great battle. The end.
Please pardon me for this rather brusque outline. It does not do justice to the magical writing in the book. But let us not beat around the bush. C.S. Lewis is trying to convey what he sees as the wonder of Christianity to children. The story of the coming of a savior, who allows himself to be killed to save the blood of a sinner, and who finally rises from the dead to destroy the evil, is, in no uncertain terms, a Christian tale. This is made clear in the third book, The Voyage of the Dawn Treader:
"You are too old, children," said Aslan, "and you must begin to come close to your own world now."
"It isn't Narnia, you know," sobbed Lucy. "It's you. We shan't meet you there. And how can we live, never meeting you?"
"But you shall meet me, dear one," said Aslan.
"Are — are you there too, Sir?" said Edmund.
"I am," said Aslan. "But there I have another name. You must learn to know me by that name. This was the very reason why you were brought to Narnia, that by knowing me here for a little, you may know me better there."
That is no reason to particularly like or dislike the book, regardless of religious beliefs or not. The story is exciting, the writing is excellent, and as Sontra points out, it is never patronizing to the children for whom it was intended. I would never hesitate to recommend this book to anyone.
The book was the first in the series that became known as The Chronicles of Narnia. With the exception of a few of the later books, it is the book with the heaviest allegorical content. They are, nonetheless, all intelligent, and, in my humble opinion, some of the best fantasy novels ever written.