I don't have anything prepared for this morning - so, so very tired - so I'm borrowing a piece on terminology that I wrote for something else. And now I must get to wor... zzzzzzzzz
I'd love to tell you that this is going to be definitive, but it's not. These are terms that people use in different ways, with different meanings, and any attempt to absolutely define them is pretty well doomed to failure. However, there are some distinctions that I want to point out here because I think they'll be helpful.
Agnosticism: basically, this is the position that we don't know (or can't know, which is sometimes called "hard" agnosticism) whether or not God exists.
Atheism: this is either not believing that God exists (called "soft" atheism) or the belief that God does not exist (called "hard" atheism).
Antitheism: this is the belief that religion is inherently harmful; generally, that it slows and suppresses social, scientific, and technological progress; that it discourages critical thinking, etc. etc. ad nauseum.
I'm breaking these down this way because it's very easy to confuse these qualities, or to think that they're all basically the same thing. The most common misconception I see is the idea that all atheists are also antitheists. That really isn't the case, but it's an easy mistake, since the most vocal atheists are vocal precisely because they're also antitheists - and also because a lot of newly deconverted atheists go through an angry-lashing-out stage as part of the process of letting go of their former beliefs. (This has been compared to the tendency to demonize ex-wives and ex-girlfriends after a breakup, and I think the analogy is apt.)
The second most common misconception I see is that agnosticism is somehow better or gentler or more honest than full-scale atheism. And, actually, there is some truth to that, particularly because the sorts of people who don't believe in God, but also don't think religion is necessarily evil, are often a lot more comfortable referring to themselves as agnostics than as atheists. (In fact, up until a couple of years ago, I did that myself.)
Mainly, though, there's a lot more overlap between atheism and agnosticism than those categories would tend to indicate. As a practical matter, it doesn't make much difference whether someone doesn't know whether God exists (and so carries on their life as if He didn't), and someone who believes that God doesn't exist (and so carries on their life as if He didn't). Looking at that another way, most atheists - if they're being honest about it - will freely admit that they can't be absolutely sure that God doesn't exist. I've only met a few "hard" atheists, and for them the accusation that they rely on faith just as much as believers seems perfectly legitimate. That's not to say that there aren't any hard atheists out there; I'm sure there are. But most of the forceful "I'm absolutely sure that God does not exist" opinions are part of the rhetoric, rather than cogent philosophical positions.
...Which brings me to my last point: the idea that atheists are secretly just antitheists. Basically, this view says that atheism is just a smokescreen - that the so-called "atheists" really do believe in God, they're just angry at Him or in rebellion against Him. This is, as far as I can tell, absolute nonsense. I mean, I'm sure that somewhere out there, there exists a pure antitheist: someone who, to quote from Pitch Black, absolutely believes in God and absolutely hates the M--- F---. But I've never met anyone like that, in any context. (And, really, is there any meaningful way to oppose an all-powerful being?) For most atheists, the belief that there is no God is not really a choice; it's a conclusion, the best they can come to from their experience and other evidence.
There's probably more to add to this, but I think this is a decent start - and it's certainly long enough already.