He's as much a theologian as a philosopher. His writings rejected Hegelianism (which itself was more or less a derivative of Immanuel Kant's ideas), and basically rejected the Church of Denmark, which he viewed as obsessed with ritual and dogma, at the expense of spiritual enlightement. He was probably the first true existentialist.
Cassie, you should read Either/Or, it's widely regarded as his masterpiece. It's very dense and hard to read (Kierkegaard himself refused to give meaning to his work, and wanted it to be "difficult"). Anyway, it basically deals with the idea of the asthete vs. the ethicist. It's a response to Aristotle's question, "How should we live?" I.E., what is the meaning of life. It deals with the essential dichotomy of human nature: Should we do as we please, indulging every whim, or should we be bound by the conventions of society, and subsume certain desires for the "good" of our fellow man, and to be "moral" and "ethical". From what I remember, S.K. does a pretty excellent job of defining just what it means to be moral and ethical.
But, by no means is his work a "beach read"; it's really best read when you have an annotated version or are in a class, like I was.
