Because questions like this are so common on Quora I wrote a whole FAQ
over there to cover it. Here's the summary:
Atheism is the lack of belief in any gods. Agnosticism is the recognition that the existence of gods is an unanswerable (because it's untestable) hypothesis. They really answer two different questions. Ask me what I believe
and I'm an atheist. Ask me what I know
and I'm an agnostic. Some people like to use agnostic to mean "undecided", but that's not really a very good definition. If you don't answer the question "do gods exist?" with a yes, you're an atheist.
Note that atheism is not
a philosophy or a movement. It is merely the lack of belief in gods. It has nothing to do with morality, good or evil, or anything else. It's just the gods thing. Really.Humanist
is the answer to yet another question: What is your philosophy or ethical stance? This Wikipedia entry
is a good primer, but essentially it means one looks to human agency and science for knowledge, not religion. That's why it's sometimes known as secular humanism.