The more obvious meaning of Atheism is the absence or conscious rejection of a faith and belief in God or Gods. Atheists do not conform to the belief that we were created by God or that the entire Universe was created by God.
All religions believe that God is all-knowing, all-powerful, all-forgiving, and all-loving. These are the qualities that distinguish Gods from humans. Atheists argue that evil doesn’t just exist but abounds in this world and a God who is all-knowing would be aware of its existence and do something about it. After all an all-powerful being would be able to eliminate evil and an all-loving being would want to put an end to the misery suffered by his people. The truth, however, is that evil continues to prevail, good men continue to die and the bad men walk around free. Hence, God does not exist. It is as simple as an algebraic equation.
Atheists believe that the universe is a huge place governed by natural laws that do not make allowances for either our desires or our sufferings .Assuming that a 9/11 happened to America because God wanted to teach the Americans a lesson, or that the Tsunami wiped out many Muslim dominated areas is absurd. We may not be able to undo the past but we can take precautions that it does not repeat itself. So, instead of waiting for divine providence to intervene, we can and should do all in our power to make this world a better place. We are all in this together for better or for worse. We have to look out for each other. It is our moral duty, when we see our brethren suffering to reach out to them the best we can. That is what Atheism is really about.