What is written below is a synopsis of how
Carl Sagan explained the two in his book,
Contact
Given: Atheism is the belief that there is enough evidence to prove that God does not exist.
Given: Agnosticism is the belief that there is not enough evidence to prove that God does exist.
The late Carl Sagan argued through a fictional character in his novel, Contact, that atheism and agnosticism are practically the same concept. To be a proclaimed agnostic, a person must be able to envision a reality in which God does not exist. An agnostic would have to admit that our universe could very well be such a reality. However, an agnostic would also have to admit that our universe could very well be one of the God-inhabited realities. Therefore the difference between atheists and agnostics is that agnostics do not necessarily accept our reality as being one without God.
In the time since I have written this node, I have come to the conclusion that it is wrong. I will leave it here because I still think it has some merit, but if you want the real skinny on the deal, listen to Shadownode or tailchaser. They won't steer you wrong.
Yo Shady, Know me as Ulysses.