This has long been a frustrating issue for me personally, because it seems the general public has a very poor grasp of the terms and wrangling over them is quite a pastime among atheists, it's basically the atheist version of the hat dance. In particular, there are a few train wrecks floating around in the blogosphere about it, like this one.
So I'd like to take a few minutes to explain what they mean and help clear up misconceptions.
Among the public, it's not uncommon to see atheism defined as dogmatically "denying" God and agnostics defined as metaphysical fence-sitters who give equal credence to both atheist and theist claims.
In actuality, it's really very simple.
Do you believe in a god?
Can one know whether a god or gods exist?
(author's note: I'm using "gnostic" here as a polar opposite of agnosticism for convenience's sake. The belief that God can be/is known has generally been expected as a default belief of the religious group, and therefore a separate terminology has never really been needed)
It gets a little trickier than just that, because the more observant reader might respond, "Which god are we talking about here?" Are we talking about the Christian god or the Deist god or the Greek gods? Further, shouldn't "God" be properly defined before I answer? Excellent points, and I wholly agree. But assume for the moment that your generic, all-powerful creator God is meant.
Atheism is about belief, agnosticism is about knowledge. Since they answer different questions, they're not necessary mutually exclusive. In actuality, agnosticism is not a third path between atheism and theism. Either you buy into theistic claims or you don't. If you do, you're a theist. If you don't you're an atheist. You might be agnostic in addition to being theistic or atheistic, but you can't define yourself as a non-theist non-atheist any more than you can pick a creamer that isn't dairy or non-dairy. Whatever your stance may be, at the end of the day, you either worship and believe in a god or you don't. Atheism is merely a broad category of people who take the latter approach. (Right about here is where the hate mail floods in from people who aren't theists but intensely dislike the atheism label, often narrowly defining atheism with traits they dislike, like atheism's perceived arrogance or alleged god-denying omniscience)
Think about it this way: do you believe in unicorns? Nope? Well, do you know whether or not they exist? You have at least two options here: You could claim to not know for sure whether or not unicorns exist, but you don't believe in their existence, at least not without solid evidence that they do exist.
Another option is that you could claim that you know that unicorns don't exist because the surface of the Earth has been pretty thoroughly mapped out with no unicorns in sight. But then the unicorn believers claim that unicorns are adept hiders who only reveal themselves to the faithful, or exist in an alternate dimension and only project themselves into our world for a brief time, etc, etc. Wow, you're quite the arrogant sod for claiming you know they don't exist, aren't you? And thus, you're forced to concede the possibility, however slight, after being bombarded with a series of increasingly unfalsifiable and dubious claims. Meanwhile, unicorn believers are quite free claim to "know" unicorns exist without much examination. Dreams, football victories, natural disasters, and unicorn-shaped toast are all taken as proof of unicorns. That's essentially how existence of God arguments play out - double standards galore.
As for myself, my own views closely match agnostic atheism. I don't "know" whether or not gods exist, but I don't have a single good reason to suppose that any of them do exist, so I'm functionally atheistic regarding any conceivable god. But show me credible evidence to the contrary and I might change my mind.
Is this arrogant atheism? Hardly. What is arrogant is blind belief in a god parading itself as informed by knowledge when it clearly isn't.