I really does depend. What Jag says is right, though folks will jump between what they mean be 'atheism' and I can definitely understand why it can be confusing it and it's why I try to avoid it as much as possible.
By simple definition all 'atheism' is, is the lack of belief in any deities. That is it. It's nothing more, it is nothing less. Atheism is not a 'proper noun' so it should really only be capitalised when it is grammatically correct. Normally I'm not a grammar Nazi, but when it comes to terms like 'atheism', the difference in the 'A' can imply two very different things and give people the wrong idea. For example, I am a dentist and I am a Dentist. The former suggests that my profession is in dentistry, where as 'Dentist' suggests I am a part of some kind of organisation, a specific group of people united by the certain standards of dentristy, perhaps something like the British Dentistry Association. Atheism is like the former, to be an atheist you're not a member of 'anything', so atheism doesn't need to be treated like a proper noun. If you're a Christian, you may not necessarily belong to any church but you are the member of a religion.
However, there are atheists united by similar thinking and similar ideals and for me, but I don't think there's actually a term to group them, they may be referred to as 'atheists' and you might find 'Atheism', with that capital 'a' to be fitting, giving it a double meaning, but I don't think it has quite entered the English language as such and if 'Atheism' were to become an organisation, I think I would find a new way to describe myself, whilst I respect many atheist peers, I don't want my lack of belief in any deities to be organised. I guess it's the same way some theists feel about organised religion.
Personally, as far as atheism goes, I am agnostic atheist. But I do not agree with the terms 'soft' and 'hard' atheist, I think it applies well in philosophies like determinism, because you could argue different levels of determinism. Of course a hard determinist doesn't believe free will exists. A soft determinism believes free will exists to a degree but the degree can vary depending on who you ask. For atheism, gnosticism and agnostism are pretty binary, it's "do" or "don't", like atheism and theism. Gnostic = with knowledge, agnostic = without knowledge, you can't "partially know" just as you don't "partially believe".