A theist, or believer in God, will tell you that he ABSOLUTELY exists. An atheist (non-believer) will say he ABSOLUTELY doesn't.
Well, the last time I checked, it's been like 2000 years since God was reported to have talked to anybody, making it pretty difficult to prove. Besides which, all religion is predicated on faith (See also: Not supposed to be proven).
Prove he doesn't exist? Possibly the only thing harder to do. Besides the enormity of the task, there's the fact that anyone who ever took a logic class knows and that is that you can't prove a negative. You can only prove (sometimes) that something hasn't happened YET. It may still the next time.
So... you have a theist, an atheist and an agnostic... None of them really knows the truth about the nature of existence.
But only the agnostic is smart enough to admit it.
Most Christians will get very defensive if someone challenges that God exists, while some Atheists will treat every person who believes in God as if they are wrong, and there is no Gd and anyone who believes there is is stupid. Not all Atheists are like this, though.
A lot of Christians dislike Atheists and Agnostics because they have such a strong belief in God that they believe they know something for sure that no one can. What these people need to realise is that there is a very great gift that God, if there is one, gove us...free will. He gave us a brain which can think for itself and believe what it wants to...why wash it with things that some book or preacher say and make yourself believe it's the truth?
Atheist's response-You're stupid. There is no God. Religion was created by man, everything in The Bible is a lie.
Agnostic's respose-You have know way of knowing that for sure. There may be a God, there may not, we will only know for sure when we die.
Atheist-There is no God.
Religious nut-There is too! The Bible says so! You will go to hell for saying there's no God!
Atheist-You're stupid, there is no God, religion is a myth and was created by man.
Agnostic-There could be a God. It's not dumb to believe there is a God, it's not bad to believe there isn't one. Agnosticism is the best way to go!
Person B: I'm an atheist and I hold this same view, as Person A's agnosticism doesn't assert the existence of any gods. In this case my atheism is the same as Person A's agnosticism, though if I was a strong atheist I would still be an atheist, but not an agnostic.
Person C: I'm an agnostic theist and I assert that a god exists, though I'm not entirely sure. My agnosticism is different from Person A's an Person B's as I assert the existence of at least one god.
Person D: I believe it's impossible to know if any gods exist, thus I am either theistic or atheistic in my views on the existence of god as a theist could believe it to be impossible to know if god(s) exist, just as an atheist could assert it's impossible to know if god(s) exist.
Atheism: "I know for certain that there are no gods"
Agnosticism: "I only know that I don't know"
Either used to show a person as "sitting on the fence" or somebody who is certain that the existence of a supernatural being can't be proven or disproven.
Agnosticism is merely the "ism" for "Agnostic".
Atheist: *brain broke*