It is a very common mistake to confuse atheism with certain beliefs. Atheism is by default simply the lack of belief in a god, that is it. It's not even a belief, it's the lack of a belief. Anything else that someone believes in is down to the individual atheist and is not representative of atheism as a whole. Here are some things which commonly get mistaken for or added onto atheism. Any atheist may also adopt any of these positions, where possible, but you should not assume automatically that they will.
A religion
Atheism is not a religion. This is a false description given sometimes as an honest misunderstanding, and sometimes as a way to try and equate atheism with religion. Atheism has no prescribed books, no dogma, no membership requirements, no organized structure, no instructions, no leaders, no rituals... nothing. Anything any particular atheist decides to do or believe is purely their choice. For this reason, trying to blame anything on "atheism" is an entirely fruitless endeavour. There is nothing to blame it on. You can blame it on a particular group of atheists who happen to all agree on a further issue, but you can't blame it on "all atheists". As there are no leaders being worshipped and no books or other things being put forward as "special", atheists are not responsible for the actions of other atheists, unless they are directly involved. Atheism makes no claims of knowledge, and does not try to "replace" religion with anything. If someone leaves their religion and becomes an atheist, what they replace it with is entirely up to them.
Anti-theism
This is a position where you are in some way against organized religion. You may dislike it and want to see aspects of it changed, or you may find it so repulsive that you want it stopped altogether. However, just being an atheist does not automatically make you an anti-theist. You can be an atheist who is perfectly happy for religion to carry on as it is. Even a theist can be an anti-theist, in that they may feel organized religion has problems which need addressing.
A belief system / world view
Atheism is neither of these. To know someone is an atheist tells you nothing else about them. They could be a scientist who is against guns and thinks morality comes from within us; or they could be a spiritualist who thinks guns are great, and our morality comes from our shared auras. An atheist can believe anything else, so to assume anything further about their beliefs is to make a big mistake. If in doubt, just ask any particular atheist what their beliefs are.
Gnostic atheism, or the belief that there is no god
These are two claims further to atheism. The first is a claim to know there is no god. The second is to express the belief that there is no god; this is addressing a seperate claim. To be an atheist requires neither of these, although any particular atheist may also take these positions. If someone is really unsure and does not know what to believe, then they are neither accepting the proposition that there is a god, nor are they accepting the proposition that there isn't. They are saying they don't have enough evidence to be convinced either way. This is a typical description of someone who identified as "just an agnostic", preferring not to label themselves as an atheist, even though they are.
As another example, if I am about to flip a coin and I ask you whether you believe it will be heads or you believe it will be tails, the honest answer is that you don't believe either. You don't have enough information to make a meaningful judgement. Refusing to accept one claim of knowledge does not require belief that the opposite of that claim is true. There is no requirement to pick "one side or the other", it's perfectly fine to say you don't have a belief either way. You would literally just be guessing, so what is the point of that?
Secularism
This is the position where you believe everyone should have religious freedom, whatever their religion, within the law. The governing bodies are not to favour any one religion, other theists, or atheists. This means there cannot be a favourite or endorsed religion, nor any discrimination against any theists or atheists. If you believe in this kind of system, you are a secularist. Many countries have this kind of government including (most of) Europe and America. However, this has nothing to do with atheism. A theist can be a secularist, too. In fact, any theist in a secular country is only non-secular if they are against that form of government and think their religion is due special treatment, or should actually be running the country in a theocracy. You can have a country composed entirely of theists, of as many different religions as you like, and it can be a secular country. It's probably fair to say most atheists are secularists, but it's not automatic so shouldn't be assumed.
Science
It is often assumed that if you are an atheist, you must be a firm believer in the scientific method, and believe standard scientific theories such as evolution. This is not the case. You can be an atheist and know nothing about science, or even reject it entirely. You need not believe any theories, in fact you can also believe in the supernatural. Again, probably most atheists tend to be sceptics and as such respect science, but this should not be assumed.
Communism / Nazis
This may sound ridiculous, but yes, I have heard atheism compared to both these things and talked about as if one leads logically to the other. No. Atheism is not any kind of political view. It is not the view that religion should be abolished, it is not communism or being a nazi. It is not calling for violence, or calling for the control of populations through any method. Any particular group of atheists may hold certain political views, but that is their personal opinions and is not representative of atheism.
Satanism / hating god
Mainly due to the propaganda of the church, some people think that to not believe in god means you in fact worship Satan. But almost certainly, if you reject the Christian god then you reject Satan as real also. And not believing in god does not mean you hate god or are angry at him; that makes no sense, you cannot be angry at something you think doesn't exist. You could be angry at the fact that other people think he exists, or you could say that you would hate him if he exists as described in the Bible.
In fact Satanism has developed recently as a religion, but it is (in my opinion) mainly satirical. It is composed mainly of atheists and they chose the name in order to demonstrate the arbitrary nature of religion. They aren't actually worshiping Satan. If you ask that Christianity be respected, then to be fair you should also respect any other religion. They often take actions that mirror those of Christianity to make this point. For example, if Christians have a religious monument somewhere and demand that it remains, then Satanists will apply to add their own monument. If the Christians object, then this highlights the fact that they want religious favouritism.