C. S. Lewis was an atheist turned Christian due to a number of events that happened within his life. Because he was formerly an atheist many of his writings are defenses of the Christian faith in which he attempts to disprove the very logic that he once held.
Lewis was Anglican in denomination, the best book that seems to be the most clear on his theological views is Mere Christianity. One of the main ways in which many "theologians" today disagree with Lewis is on his view of election. Lewis believed that people have the freedom of choice when it comes to salvation, whereas many today lean toward a Calvinistic or Reformed view which essentially says that when God calls a person to salvation they do not have a choice, they will always accept. This is from the Irresistible Grace part of the Calvinist TULIP theology. Another way in which he differs from many evangelicals is that in his book The Great Divorce Lewis seems to believe in purgatory in some sense. It is hard to know exactly what his views are as many of his works (like the Chronicles of Narnia) are predominantly allusions and metaphors.
The best way to know what Lewis believes is to read his apologetic books such as Mere Christianity, The Problem of Pain, and others. His book called Screwtape Letters also shows many of his beliefs in a narrative form.
C. S. Lewis was a profound Christian thinker. The odd thing is that by training this was not a theologian but a philologist and Medieval scholar.
In terms of his theological perspective, we can say that he was a traditional Protestant. In other words, he embraced traditional Orthodoxy. This is why so many evangelical and traditional mainline denominations claim him as their own.
The best way to get into the heart of C.S. Lewis's theology is by reading his slender volume, Mere Christianity. In it, he expounds his understanding of Christianity. This is not to say that he did not have non-tradtional elements in his thinking. For example, he is more inclusive than most conservative theologians.
We should also mention that he is an apologist. This means that he is one who defends the Christian faith or makes it understandable for people who have questions about the doctrines of Christianity.