Atheism I would say is the belief that there is no God. Or since that wording is so offensive to some people, the lack of a belief in a God.
Being areligious sounds more like lacking a religion to me. And since they don't have a religion, they probably just consider themselves to be a "don't know, don't care," kind of person. But that "don't know," part of it might make them qualify to be considered agnostics. But it's debatable.
