As understood within the philosophical tradition, "atheism" is contrasted with both theism (the belief that God does exist) and with agnosticism (the belief that it is impossible to know for sure whether or not God exists). Atheists, agnostics, and positivists (who believe that "God"-talk is nonsense) can generally be described as nonbelievers.
The above broad definition of atheism does not distinguish between "strong" and "weak" atheism, which is commonly drawn within the freethinking? tradition (as distinguished from the philosophical tradition). "[Positive atheism]?" is the positive assertion (or belief) that God (or some gods, or all gods) does not exist--that is, a positive atheist may assert that the Judeo-Christian God does not exist, or that all gods do not exist. "[Negative atheism]?" is used to describe someone who does not have a God-belief or someone who is not a theist, a position which is only slightly different from Agnosticism. Furthermore, an atheist can be both a positive atheist with respect to the Judeo-Christian God and a negative atheist with respect to any other gods.
See the rationality of atheism for further discussion.
Atheism is generally misunderstood in many societies and those who openly express their non-theistic points-of-view are often mistreated, ostracized and the subjects of discrimination (as is often the case for adherents to minority points-of-view in many parts of the world). Like their religious counterparts, many atheists align themselves with organizations that express or support a non-theistic worldview.
--- See also: secularism, philosophy of religion, the rationality of atheism, listing of noted atheists, agnosticism, naturalism?, theism, deism