In my travels as president of the American Humanist Association, I am often asked to explain the difference between atheism and humanism. Since the question gets raised so frequently, I thought it might be a good idea to provide a short explanation here.
To understand the difference between the terms atheism and humanism, realize first that the former refers to a view of only one specific issue (the existence of gods) whereas the latter is a broad philosophical outlook. From that premise, the rest falls into place easily.
When Sally describes herself as an atheist, she is revealing only one fact about herself: she does not believe in any gods. Note that she is saying nothing about other supernatural beliefs, and she is saying nothing about her ethical/moral principles. Although atheists, being without any god-beliefs, usually do not accept other supernatural claims (such as belief in astrology, reincarnation, or life after death), technically Sally could believe in such notions and still wear the "atheist" label. Moreover, while some might be inclined to make certain presumptions about Sally's ethical principles upon learning that she identifies as an atheist, such presumptions, based on her atheist identity alone, are unwarranted. Because the atheist identity refers only to the singular issue of god-belief, it says nothing about her moral stature, good or bad.
When Patty describes herself as a humanist, on the other hand, she tells us numerous things about herself. For one, she tells us that she approaches the world from a natural standpoint, meaning she rejects all supernatural beliefs, not just the singular issue of divinities. In seeking truth and knowledge, she accepts empiricism, science, and reason as her guides. Identifying as a humanist, Patty is declaring that she holds certain values, including a support for human rights, peace, democracy, and personal liberty with a sense of social responsibility. These principles are subject to some interpretation, of course, and humanism rejects outright the notion of dogma, but the general thrust of humanism is a progressive, forward-looking lifestance that encourages creativity, critical thinking, and personal fullfillment within the context of social well-being. The AHA sets forth a vision of humanism in its document Humanism and its Aspirations, which has been signed by 21 Nobel Laureates. The International Humanist and Ethical Union also has a short statement of humanist principles called The Amsterdam Declaration.
The atheism/humanism comparison shouldn't be seen as an either/or situation where one must choose sides. Many humanists, but not all, also identify as atheists; many atheists, but not all, also identify as humanists. For many years I identified as a humanist but not an atheist, much preferring the broad philosophical label of humanism to the more narrow definition of atheism. In recent years, however, I've come to the opinion that the "atheist" label is wrongly stigmatized in American society, so nowadays I'll also identify as an atheist mainly to push back against the unfair prejudice. My humanism is more important to me than my atheism, but I realize that the Religious Right draws much strength from marginalizing atheists, so we're doing a service if we can help the public to realize that atheists should not be feared.
This brings me to my gentle criticism of Nigel Barbers's various posts on "Why Atheism Will Replace Religion." As an activist in the secular movement, I'm hopeful that Barber's general vision, of a more humane world where dogma and superstition dwindle in importance, is correct. I would simply point out that, if this comes to be, the important element will be the broad, affirmative values of humanism, not a singular notion of nonbelief.