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 a primitive inability to believe religious teachings that 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 those 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 or disbelief in the existence of God. "Strong atheism" is an explicitly held belief that God does not exist. There is a qualitative difference in the "strong" and "weak" positions; it's not just a matter of degree. Some atheists believe in the non-existence of all Gods; others limit their atheism to specific Gods, such as the Christian God, rather than making flat-out denials.