So, I am an agnostic atheist. You might be confused by that statement. Aren't agnostics and atheists different things? Agnosticism and atheism answer two different questions. I will clarify that here.
Agnostic literally means "without knowledge", or more colloquially, "I'm agnostic" means "I don't know". This is usually used to answer the question "Do you believe in God?" Most of the time this is used to avoid an argument with your religious relatives and coworkers. However, "I don't know" is a silly answer to "Do you believe in God?” How does someone not know if they believe in God? If you do not know what you believe, who does?
Agnosticism is an answer to the question "Does God exist?” One can reasonably answer "I do not know" to that question. On the existence of God, I am agnostic. There might be some God somewhere that I do not know about. By way of comparison, I am also agnostic regarding leprechauns, vampires, extra-terrestrials, and unicorns; so don't get excited. There might be extra-terrestrials somewhere, but I have never seen any good evidence. Some people claim to know that God does not exist. This is usually based on the idea that God is a self-contradicting term, like a square circle. However, I think it is mostly a matter of semantics and I dislike such games.
Atheism means "without God". It literally means that you have no dealings with gods, I suppose, but it is usually means that you do not believe God exists. I suppose that someone could believe that God exists but also not worship him, but that would be strange, especially considering the punishments at stake, but that is another topic, so I will use it as an answer to the belief question.
Therefore, when one says they are an "agnostic atheist" one is stating "I do not know for certain that there is no God, but I do not believe there is." However, this is also like saying “I do not know for certain there are no vampires, but I do not think there are."
In addition, atheism is not a belief system. It is the lack of belief. You really can't ask what atheists believe anymore than you can ask what non-skydivers believe. Other than believing that skydiving doesn't look fun, claiming to be a non-skydiver does not say much. Trying to define someone or something by what it isn’t is not a very useful practice.
When most people think of an atheist, they are really thinking of a secular humanist, or something similar. Secular Humanism is an ethical belief system that has tenants that are accepted by its members. If you had to boil Secular Humanism down to one sentence, it would be "Don't be a jerk". Look it up. However, there are atheists that are religious. There are secular Jews, secular Buddhists, secular Unitarians, etc. So Atheism is not a terribly descriptive term.