Defining any difference is rather arbitrary. The most one can say is that different organizations and terminologies have somewhat distinct historical trajectories.
Just going by the terms alone, humanism implies more of a value system than does atheism. Contemporary secular humanist is de facto atheistic, though historically the humanist movement evolved, as far as I can determine, out of the liberalization of Christianity, more specifically Protestantism, and in the USA, thanks to a great influx of Unitarians who embraced an explicitly naturalistic world view (see the first Humanist Manifesto, 1933).
Atheism terminologically does not imply as much in the way of human values, but again, anyone can define these world views any way one wishes. For example, the atheist credo as embodied in the American Atheists membership application does include what we would call humanistic values.
I think it is rather pretentious for "humanists" to think themselves superior to "atheists". Actions after all speak louder than words and calling oneself a humanist means no more than calling oneself a Christian when it comes to translating principles into action.
I don't unequivocally associate myself with humanism or atheism though I embrace both, because neither is a complete philosophy, and cannot be, as their explicit commitments only partially cover what is required of a total world view and an understanding of reality--esp. social reality--and the history of ideas.
I am an Atheist and also a Humanist. Being an atheist simply means that I do not believe in any supernatural gods or deities. Atheism is a simple lack of beliefs, nothing more, and that is exactly why it is so hard to get a bunch of atheists to work together since Atheism does not mean that people have a common political or ethical set of beliefs. Any atheists that says atheism is a complete belief system or set of ethical standards is being totally dishonest or simply does not know what Atheism means.
Humanism fills the role of an ethical system for atheists and non believers that is usually filled by religious belief systems for believers.
Honestly, I don't like the term "belief" system since reality does not require belief in order for it to exist. I also stay away from words like morality since they are themselves subjective and have no real basis in reality either. These catch terms used by theists need to be set aside and more accurate terms used by non-theists to describe our ethical basis for living.
Humanism is an ethical system and just one such which atheists can choose. Many atheists don't even take the road of calling themselves humanists or anything else since they are capable of discerning "right" and "wrong" without the need for a set of guidlelines laid out by "gods" or organizations. Let's face it laws and right and wrong are defined by humanity in order for society to exist in harmony, nothing more and nothing less. That's not to say that there shouldn't be certain rights that are considered inalienable for all human beings regardless of where you live, but in all honesty we are many many years away from getting to that point globally and probably won't see it in our lifetimes or even the lifetimes of our children.
YES Josh I definitely agree. Oftentimes, theists will attempt to use words that THEY defined, as a means for belittling or having complete control over the matter in speaking. HOWEVER, when we take a step back to acknowledge reality, as you've stated, morality as they've defined it, IS NEVER CONSISTENT with what is REALLY happening... LOVED your response there... The fact is, though they claim these definitions to be subject to themselves the FACT still remains that, they only wish to define these terms FOR OTHERS in hopes of containing this mental control that they seem to have on non-free thinkers...
OK, I should have used "world view" instead of "belief system". In any case, "humanism" seems to promise more than "atheism", though atheists have defined themselves in humanist terms, such as Madalyn Murray O'Hair's statement on what atheism stands for (though she was in fact quite rigid about identity labels).
Even so, I don't think it's the case that humanists would agree on much besides a few abstract principles, which means that "humanism" too is not a complete world view.
If you are going to define a complete world view by adherents being in agreement on a majority of things, then there are no complete world views, including religions. Adherents to different faiths can't agree on the age of the earth, whether or not evolution happened or if it did and was "guided", whether or not it's OK to kill abortion doctors or anyone else for that matter and just about every thing else. Among xtians, muslims etc, there is no general consensus on just about any topic that you can bring up.
Personally, I simply define atheism as not having a belief in god - that is what the word means and there really is nothing more to it. We are all born atheists and with a few rare exceptions, our parents try to screw it up for us and then we have to learn to be rational and throw out faith in lieu of reality. Honestly, the road to becoming an atheist is more about the strength of the individual than their intelligence or any other factors.
What I don't like about American Atheists and other "atheist centric" organizations is that they try to give more meaning to something than is there to begin with. They all forget that there really is no meaning to life, except how the individual defines it. Atheism is not a rallying point. That is why you have such a hard time getting atheists to work together - lack of belief is simply not something that unites people and that is why atheists are not a strong force politically or anything else. Atheists can only agree on their lack of belief, they diverge on everything else from gay marriage, political parties and ideologies, abortion beliefs and the rest. We don't have the same type of organized brainwashing system as believers do with some guy who wears robes telling us what some imaginary sky daddy defines as good and bad that we must follow or suffer the circumstances. Instead we are left to decide for ourselves, based upon information and facts, what we think is the right stance for each and every issue before us. And for my part, that is a good thing since the dude wearing the robes is usually that last guy that you want to go to for advice on what is right or wrong.
There is no inherent reason why atheists should agree on gay marriage or any of these other issues. But practically speaking, there is a fair amount of convergence on a number of these issues, because liberal democracy itself is headed in that direction.
More vice versa, I think: atheism is often the result of rational thought. That's not always the case, though. Some people are raised atheist without ever having given it much thought, others are drawn to atheism by life stances or world views that appeal to them. Humanism can be one of these, but so can Raellianism, Scientology, and communism: all atheistic.
Thankfully we seem to draw most of our new godless from the ranks of rational freethinkers such as yourself.