It might seem silly to an Atheist that the concept of atheism needs any explanation at all. While it is simple the Atheist must remember that around 80% of the worlds population is a Theist of one flavor or another. Because of that, the same thing happens to Atheism as happens to anything people don’t understand. They make assumptions about it and then in a lot of other belief systems get lumped in with Atheism.
The simple definition of Atheism is a belief that there is no God. It is not an indictment of those that do, they just believe that there is no evidence to support the concept. In that way it has a somewhat neutral view of a God. An Agnostic would be a true neutral with Atheism being neutral strongly leaning to the negative.
That is the simple definition of what they are but what are Atheist not but sometimes thought to believe.
One group that gets lump is with the Atheist are those that believe in Pantheism. This group believes that nature and god are the same thing. Yes, things occur naturally but that is though god and his work stops there. Pantheist do not believe in a personal god in the way that Christians, Jews, and Muslims do. They are not Theist so in to the Atheist pile they go.
Then you have Deist. The Deist basically believe in a god that started the universe. That’s it. That was all he did or is going to do, This is not the prayer answering sort of god.
Last, and hardest to understand, are the Anti-theist. In someways they are like Atheist in that they do not believe in god but they take it to the next level. They don’t believe in god, but if there was a god, which they will not concede, it would be a very bad thing. So the long and short of it is it that the thing that does not exist is evil.
Those are the three main groups that are sometimes called Atheist but are not. For the most part, if a large booming voice spoke to every one at once in a tongue they could understand, a true Atheist would just shake his head and say “Wow, I did not see that coming” Until that time however, it is doubtful that most will see the evidence needed to change their mind.