Naturalism is an alternative to supernaturalism, which includes theism. Paul Draper, an agnostic philosopher at Florida International University, explains the difference between naturalism and theism well:
“Naturalism and theism are powerful and popular worldviews. They suggest very different conceptions of the nature of human beings, our relationship to the world, and our future. Though I hope that theism is true, I believe that it faces a number of evidential problems, problems that prevent my hope from becoming belief. …
“By ‘theism’ I mean the hypothesis that God is the creator of the physical universe. I take the word ‘God’ to be a title that, by definition, can be borne only by a perfect supernatural person. To claim that God is a ‘person’ is to claim that God performs actions and has beliefs and purposes. ‘Supernatural’ persons are not natural–they are neither a part nor a product of the physical universe–and yet they can affect natural objects. A ‘perfect’ person is, among other things, perfect in power (omnipotent), perfect in knowledge (omniscient), and perfect in moral goodness (morally perfect). While some have dismissed this conception of God as religiously insignificant, I am convinced that, for millions of Jews, Christians, and Muslims, factual belief in a supernatural person is essential for making sense of their forms of worship. By ‘naturalism’ I mean the hypothesis that the physical universe is a ‘closed system’ in the sense that nothing that is neither part nor a product of it can affect it. So naturalism entails the nonexistence of all supernatural beings, including the theistic God.“
— Paul Draper, “Evolution and the Problem of Evil” in Philosophy of Religion (ed. Louis Pojman, Wadsworth, 1997), pp. 219-220.
Thus, just as supernaturalism is broader than theism, naturalism is broader than atheism. Someone can be an atheist without being a naturalist. In other words, someone could believe in the supernatural without believing in God. I have chosen to adopt the label of a naturalist in order to emphasize that I do not believe there are supernatural beings of any kind. (Is that even possible?)