I am indebted to "reme1" at Yahoo Answers for this answer. In turn some of the text is from Wikipedia. I find it a helpful description:
Secular humanism is a humanist philosophy that upholds reason, ethics, and justice, and specifically rejects the supernatural and the spiritual as the basis of moral reflection and decision-making. Like other types of humanism, secular humanism is a life stance that focuses on the way human beings can lead good, happy and functional lives.
The term "Secular Humanism" was coined in the 20th century to make a clear distinction from "religious humanism". A related concept is "scientific humanism", which biologist Edward O. Wilson claimed to be "the only worldview compatible with science's growing knowledge of the real world and the laws of nature".
Atheism, as an explicit position, can be either the affirmation of the nonexistence of gods, or the rejection of theism. It is also defined more broadly as an absence of belief in deities, or nontheism.
Many self-described atheists are sceptical of all supernatural beings and cite a lack of empirical evidence for the existence of deities. Others argue for atheism on philosophical, social or historical grounds. Although many self-described atheists tend toward secular philosophies such as humanism, and naturalism, there is no one ideology or set of behaviours to which all atheists adhere; and some religions, such as Jainism and Buddhism, do not require belief in a personal god.
I have met some Humanists who still profess a belief in the supernatural. People often use the terms atheism and humanism synonymously (including me sometimes) but there is a difference.
Some use the atheist label as a statement of their intent, while others use the humanist label as a confrirmation of their belief in the betterment and empowerment of humankind in the here and now, rather than worrying about the existence of a hypothetical God.
Underneath we all share our common heritage as humankind!