What is atheism?
Atheism is the absence of belief in gods. An atheist, therefore, is simply someone who does not believe in gods. An atheist may actively believe that the idea of a god is meaningless or self-contradicting, or may think the hypothesis that there is a god is simple not borne up by evidence; such atheists are sometime called "hard atheists", since their atheism takes the form of an active belief in the nonexistence of gods. Other atheists believe that the question of whether a god exists isn't one that is properly formed, or that the topic isn't one that can be investigated by human means; consequently, their belief is that it is unwarranted either to accept the idea that gods exist or that gods do not exist (such a position is often called "agnosticism"). Finally, the word atheist may be used to describe someone who simply lacks belief in gods. If you were to ask such a person about their atheism, they might say to you: What's that? An atheist? I am no such thing! Yet if they happen to lack belief in the existence of the Norse god of thunder, it is only accurate to describe such people as atheists with respective to Thor. When understood this way, it can be easily seen that most every person is an atheist, to some degree. After all, most all of us lack belief in the great majority of gods that are or have been said to exist.
Isn't atheism kind of.... arrogant?
Some people dislike the term "atheist" because they mistakenly think it conveys a certain sense of overconfidence. In fact, saying that someone is an atheist says nothing about the certainty with which that person holds his or her disbelief, or the thought process that brought them to that position.
Here's an analogy. Saying that you believe a ball will fall to the ground when you drop it does not mean that you have faith in the theory of gravity -- it means only that you feel that you have warrant to make such a claim. And if you're a reasonable person, you'd be open to revising the nature of your beliefs about gravitation and falling objects, if new evidence were brought to light that warranted such revision. To take another example, it is hardly a controversial claim to say that phlogiston does not exist; nor could such a claim be called an expression of arrogance or undue conviction. So it is with atheism! If an atheism says she is unpersuaded by claims that the gods described in the Koran, the Bible, the Torah, the Vedas, or any other sort of scripture, are real and existing beings, it is not quite fair to say that she is being arrogant, dismissive, or scornful of theists. She has every right to evaluate the evidence and arguments for the existence of gods, to her own satisfaction, and according to what rules of reasoning she feels appropriate. The same right is enjoyed by believers when they make their own claims to belief (in, say, the divinity of Jesus) or disbelief (in, say, the existence of Thor). The idea that atheism is mainly an expression of hubris, or arrogance, or anger, is really nothing but a form of prejudice.
Really, knowing that someone is an atheist tells you nothing about his or her character, temperament, or ethics. Some atheists are philosophically oriented and enjoy conversation; others are argumentative, and like to go toe-to-toe on topics like ethics, science, politics, and culture. Others are uncomfortable with confrontation and may not even mention the fact that we are atheists except to very close friends. Atheists are musicians, lawyers, pastors (yes, really!) doctors, taxi drivers, teachers, politicians, soldiers, volunteers, computer programmers, yoga instructors, and on and on; they are old and young; they work and live in every national and ethnic community; they raise families; and they vote; and they matter.
Since atheists are such a diverse bunch, you will find many different descriptions of what it is like to be an atheist. For other perspectives and more information, see the American Atheists About Atheism page, our links and resources page, and read more about our Boston Atheists community.