As an atheist, my view is that 'God' is something that exists only in the minds (or hearts or souls if you prefer) of people that believe in God.
God is something that is defined only by the religious texts that promote God. If I say to a Christian "Don't you find it amusing to worship a God who has an elephant's head and eight arms?" they will tell me that their god does not because it says in the Bible that god made man in his image, therefore god looks like a human being. Likewise, every aspect of how and what they believe god to be is defined by their interpretation of the bible.
This creates a further problem in that there are so many ways to interpret the bible: very few people who call themselves Christians believe that every word of the bible is absolute truth. Most believe that many of the stories, particularly in the old Testament are allegorical rather that literal, and that god didn't actually go around killing the families of his most loyal supporter, setting bears on young children for teasing a bald man, or turning people into salt. There are, however, people who do believe that god did these things. The many Christian sects do not agree as to who/what god is, let alone when you take into consideration the lack of agreement between the three major Abrahamic religions who are all equally sure they have got it right when it comes to saying what god is.
There is no concrete way of defining absolutely who or what god is. Even if you sat down two people from the same sect and asked them to define god and how he behaves you would probably get two slightly different answers. This means that either only one person in the whole world is right about god (which says a lot for the concept of god itself if only one person will have the benefits of believing in the right way) or that everyone who believes in god is right.
The situation that you then arrive at with the second proposition is that there is no 'right' way to believe in god, and there is no way of telling how he has acted in the past or will act in the future. If everyone who believes in god is equally right in their beliefs, they must all be equally wrong. The only way that you can fix on a definition to say "Yes, this is god" is by taking a definition of one believer, or by believing yourself. That ability to define god only arises because people have beliefs. Those beliefs only exist in the mind of that believer and nowhere else. The concept of god does not exist other than by how god is defined. Therefore god does not exist other than in the minds of those who define god.
So, as an atheist, I am prepared to accept that people hold a genuine belief in the existence of god and that god exists for them. However, god does not have an independent reality that affects me in any way. For me, god has no existence.
What then does disturb me is that some people hold beliefs that terrible things will happen to me because I do not hold the same beliefs that they do (my DSis has recently seen fit to tell me I am going to hell). Personally, I find it troubling that someone would hold those beliefs when there is no need to, and think it is nothing more than a reflection of her psyche.