The simple answer is that Naturalism tells us what exists, Humanism tells us what’s important.
According to The Oxford Dictionary of Difficult Words, Naturalism is “a philosophical viewpoint according to which everything arises from natural properties and causes, and supernatural or spiritual explanations are excluded or discounted.”
So in this sense, Humanism is built on Naturalism. You could say that Humanism is naturalism plus. As the Humanist Manifesto III states, “Humanism is a progressive philosophy of life that, without supernaturalism, affirms our ability and responsibility to lead ethical lives of personal fulfillment that aspire to the greater good of humanity.”
Three Kinds of Naturalism
There are really three major kinds of Naturalism. First, there is the basic understanding of Naturalism as that which is opposite of supernaturalism. Basically, everything arises from natural properties and causes, supernatural explanations being excluded as unfruitful.