Atheism isn't a belief system.
Athiesm is the belief that there isn't a god (positive atheism) or the lack of a belief that there is a god (negative atheism, which includes agnosticism). So when you tell someone you're an atheist, you're telling them what you don't believe.
Because of the prominence of religion in history, this is to be expected. Atheism has always been on the defense, defined by its opposition to the religion of the mainstream. Religion seeks to give answers to the questions of morality, meaning and value. The 'atheist' answer implies nothing but a rejection of (at least the core of) the religious ones.
But whichever particular brand of infidelity applies to you, I'm sure you don't immediately fall to the floor frothing at the mouth or stare blankly whenever you consider a question which applies to the meaning or normative direction of your life.
Thanks to suitably broad definitions, whether you knew it or not, if you're an atheist with positive beliefs about the meaning of life, the grounds and content of morality, or the value of things, you're also a Humanist.
Humanism is simply the view that human life itself, independent of any exterior authority, is sufficient to generate truths about morality, meaning and value, and that by using reason and experience we might come to know these truths.
The fact is, 'atheist' and its cognates aren't good enough as labels for our belief systems.
You might think that we don't need to elaborate. Surely it's obvious that we have some atheistically generated beliefs about morality, meaning and value? But it's important to have a rallying point. The widespread presumption that atheism entails moral and existential nihilism drives many people to religious belief, and keeps many more people believing in god. It means that those who've recently 'converted' to atheism or agnosticism can feel lost and alone. It means that many who give a negative answer to the god-question think that's the end of the story.
Atheism isn't just the conclusion of an old argument. It's the first premise of a new one. God doesn't exist. Now what?
Humanism isn't a belief system in the sense that it offers a set of beliefs about morality, meaning and value. You won't find a list of commandments or dogmas here. Instead, it's a set of beliefs about our beliefs about morality, meaning and value. Humanism says that we don't need the threat of torment in hell, the commands of bloated institutions, or the vain authority of texts to live our lives in a loving, rational and happy way. It says that the only authority is intellectual, the only driving force for morality is our human goodness and love, and that the meaning of life is generated by the one life we have here in the universe. It's the philosophy for humanity's adulthood: it says that we can work it out on our own.
In resolving to 'work it out on our own', far from being isolated, we are united in the effort to work out and live out the vast mysteries and boundless joys of our common human-life together.
Religion has often (but not always) disdained human nature and human intellectual faculties; claiming that without god, scriptures or church, humanity is weak and lost. Humanism repudiates this, and whilst being honest about the limits and weaknesses of human nature, stresses the ultimate human responsibility to improve ourselves and others, and the exciting ability to work out the meaning and truths of human life together, as humans, using reason and experience, and while relying on our mutual love for one another.
It's of great sociological, political and intellectual importance to be 'out' about Humanism. It promotes and allows the active defense of life without god and religious authority. It supports secularism in an age when the backward seek to reverse the achievements of philosophy and science. It gives atheists a social and intellectual rallying point from which to start and return in their effort to understand life; an argumentative center; a sociological 'group' in society and history with which to share and discuss values. It challenges the notion that atheism, a term defined by opposition, involves a rejection of the questions to which religion gives an answer.
And, perhaps most importantly, saying that you're a Humanist takes religion out of the equation. No longer will our belief system be defined in opposition to another. Religion is becoming less and less relevant to the ongoing human conversation on how to live our lives. The dichotomy of atheism and theism relies, to some extent, on the ongoing relevance of theism to the human question. Humanism denies that relevance.
So next time someone asks you what you believe, say you're a Humanist.