"The way to change the world is to change people's minds. As more and more people openly discuss the fact that "God" and "Allah" are completely imaginary, the world becomes a better place. The people who believe in "religion" look sillier and sillier. Eventually, religion becomes a fringe activity that is meaningless." --
"WHAT IS ATHEISM?"
Atheism is characterized by an absence of belief in the existence of gods. This absence of belief generally comes about either through deliberate choice, or from an inherent inability to believe religious teachings which seem literally incredible. It is not a lack of belief born out of simple ignorance of religious teachings.
Some atheists go beyond a mere absence of belief in gods: they actively believe that particular gods, or all gods, do not exist. Just lacking belief in gods is often referred to as the "weak atheist" position; whereas believing that gods do not (or cannot) exist is known as "strong atheism."
Regarding people who have never been exposed to the concept of 'god': Whether they are 'atheists' or not is a matter of debate. Since you're unlikely to meet anyone who has never encountered religion, it's not a very important debate...
It is important, however, to note the difference between the strong and weak atheist positions. "Weak atheism" is simple skepticism; disbelief in the existence of god. "Strong atheism" is an explicitly held belief that god does not exist. Please do not fall into the trap of assuming that all atheists are "strong atheists." There is a qualitative difference in the "strong" and "weak" positions; it's not just a matter of degree.
Some atheists believe in the nonexistence of all gods; others limit their atheism to specific gods, such as the Christian god, rather than making flat-out denials.
"Is there such a thing as atheist morality?"
If you mean "Is there such a thing as morality for atheists?," then the answer is yes, as explained above. Many atheists have ideas about morality which are at least as strong as those held by religious people.
If you mean "Does atheism have a characteristic moral code?," then the answer is no. Atheism by itself does not imply anything much about how a person will behave. Most atheists follow many of the same "moral rules" as theists, but for different reasons. Atheists view morality as something created by humans, according to the way humans feel the world 'ought' to work, rather than seeing it as a set of rules decreed by a supernatural being.
"Then aren't atheists just theists who are denying god?"
A study by the Freedom From Religion Foundation found that over 90% of the atheists who responded became atheists because religion did not work for them. They had found that religious beliefs were fundamentally incompatible with what they observed around them.
Atheists are not unbelievers through ignorance or denial; they are unbelievers through choice. The vast majority of them have spent time studying one or more religions, sometimes in very great depth. They have made a careful and considered decision to reject religious beliefs.
This decision may, of course, be an inevitable consequence of that individual's personality. For a naturally skeptical person, the choice of atheism is often the only one that makes sense, and hence the only choice that person can honestly make.
The word "deny" can be used to mean "fail to accept the truth of." In that sense only, atheists deny the existence of god. They are not "in denial," willfully ignoring evidence; nor do they necessarily positively assert the nonexistence of god.
(Quoted from infidels.org)