Because "atheist" sometimes appears on forms asking for a religious preference, people sometimes mistake atheism for a religion. Even people who know that atheists do not believe in a god will sometimes still assume they share a common set of beliefs or principles. This is not the case.
Atheism is not an organized belief system (or "lack-of-belief system", as some may joke), but rather a word that can be associated with anyone who does not believe in any god. Because these individuals come from all walks of life, there is no reason to assume that one atheist is like another in any way, except in his or her disbelief in any god.
In some parts of the world, particularly the United States, atheists are often assumed to be scientifically minded or politically liberal. While a scientific viewpoint may lead some people to abandon supernatural belief systems, many more atheists were never religious to begin with. As to the political claim, one should keep in mind that individuals who feel underrepresented are often more liberal on average than those who do not. But, as with all blanket statements about people sharing a single characteristic, these assumptions about atheists are only stereotypes, and will very frequently be incorrect.
"Atheist" is sometimes used as a derogatory term by non-atheists. Nations and communities with high religious participation are known to discriminate against atheists, whether through legislation or more informally. For instance, a number U.S. States have constitutions requiring religious belief as a condition for public office. While it is generally believed that these clauses would be struck down by the federal courts if they were formally tested, that they remain in these documents is seen by many to indicate tacit public acceptance of discrimination.
Reasons for unequal treatment of atheists generally stem from a claim that ethics can only come from a god, and that atheists are therefore unethical. Keepers of this view would do well to consider the historical record on this matter, and to reflect on the many successful civilizations that have existed in which religions containing ethical systems were not a major cultural influence.
Atheism is often discussed in the same breath as "agnosticism", a term generally used to describe someone who does not believe in any god, but remains open to the possibility. Agnostics will often emphasize what they see as the unanswerability of 'the god question', while an atheist may not feel the question even deserves asking. These two terms often have somewhat different meanings depending on who is using them, and discussions about whether someone is agnostic or atheist will frequently prove inconclusive.