Q: Is atheism a religion?
A: Most people think of religion as involving two main things -- belief in at least one god, and a set of rituals devoted to perpetuating a given dogma. Atheism fails on both counts, since by definition it does not hold any god belief, and there are no rituals or dogmas to follow.
More broadly, of the eight definitions of "religion" given by dictionary.com, only one is even vaguely applicable to atheism: "something one believes in and follows devotedly; a point or matter of ethics or conscience: to make a religion of fighting prejudice." But even this definition is applicable only to strong atheism, the positive belief that there are no gods. Atheism as defined on this site (or weak atheism as it's called elsewhere) -- the absence of god-belief -- fails this aspect as well, since it is not a belief but a lack of belief.
Atheism by itself is not a system of beliefs. At most it is one belief, that there are no gods, and in general it's not even that -- it's just a description of an absence. Atheism can be part of a larger set of beliefs, which together form a coherent philosophy of the world, but by itself it is no more a philosophy or a religion than a foundation is a house.