Atheism and agnosticism deal with the concept "god" in different spheres. Agnosticism is about not having knowledge about god or its existence while atheism is about not having a belief in the existence of god.
Agnosticism = lack of knowledge
Atheism = lack of belief
I think that nobody has true knowledge of the existence of god. Therefore, everybody is ultimately and technically an agnostic. A form of weak atheism is also referred to as agnostic atheism. There is a middle ground between theism and atheism in the technical sense since there are those who are "undecided" and some of them prefer to call themselves among many terms as non-theist or post-theist or what-not. Agnosticism is not a middle ground between theism and atheism since agnosticism is present in the whole set. But in practice, those who are at the atheism end of the spectrum, live in a world where gods are not part of reality.