There's a hell of a lot more to it than that, actually.
There are actually seven books in the set: The Magician's Nephew; The Lion, the Witch, and the Wardrobe; A Horse and his Boy; Prince Caspian; The Voyage of the Dawn Treader; The Silver Chair; and The Last Battle. The order I listed isn't the order in which they were written (The Lion, the Witch, and the Wardrobe came first, in fact), but they follow the chronological order of the story and are the order in which C.S. Lewis said that he intended them to be read. I really do feel sorry for those who have never heard of Narnia; you missed out on something truly spectacular.
To make a brief summary, Narnia is another world, one of many like our own world was. It is a world of talking, intelligent animals, and many creatures out of mythology, such as giants, centaurs, fauns, and merpeople. Narnia was created by Aslan, the great golden lion who is the sun of the Emperor-Over-the-Sea. For the most part, the seven books tell the stories of children from our world who, through various ways, find their way to Narnia. In The Lion, the Witch, and the Wardrobe, four brothers and sisters named Peter, Susan, Edmund, and Lucy come to stay at the house of Professor Kirke, an eccentric old man, to avoid the London Blitz. While in the house, Lucy, the youngest, discovers a wardrobe during a game of hide-and-seek. Going into it, she discovers not a simple rack full of coats, but the doorway to an entirely new world. That's about as much as I can say without giving away any spoilers; the story is rather difficult to put into words; similar to Lord of the Rings, it's something you truly have to read to understand.
The Chronicles of Narnia may seem like a simple children's fantasy story on the exterior, and indeed, that's all I ever saw them as at the young age when I first read them. However, as I mentioned above, they are so much more than that. C.S. Lewis is a famed Christian apologist; his work Mere Christianity is rather well known. Anyway, the Chronicles of Narnia, in their entirety, are really a beautiful allegory of Christianity; the symbolism is present on many different levels. I won't go into it too much here, since I'll leave it to those who decide to read the books to discover for themselves. The overall theme of the story is really about the idea that evil can never really triumph, that good will win out in the end. The ending of the last book, The Last Battle, was so genuinely good that it almost brings tears to my eyes. As you can tell, I hold a deep love for these stories; they had a large impact on my childhood and really inspired my ongoing love of the genre of fantasy.
As an aside, for anyone who has read the Narnia series, I'd also recommend C.S. Lewis's Space Trilogy, consisting of Out of the Silent Planet, Perelandra, and That Hideous Strength. Once again, these books contain Christian themes, but they are definitely adult works, and I consider them to be quite good.
