In theory, yes. Try being either an atheist or an agnostic and figure out which; it's not all that easy.
First, I agree with SPMiller's definition. To add onto it:
Atheism is about the existance of God.
Agnosticism is about what we can know about the metaphysical.
Question:
Does (do) God (gods) exist?
Answer:
1. Atheist:
No.
2. Agnostic:
a) Nobody knows.
b) This cannot be known.
c) The question is irrelevant.
Since you can believe in God while accepting that there is no rational grounding to your beliefs, it's quite possible to be theist agnostic. A theist agnostic would recognise his/her own belief in God as irrational. Similarly, an atheist agnostic does not believe in God but accepts that his believe in the non-existance of God is irrational. Both theistic and atheistic agnostics would probably hold a relativist theory of what belief entails ("I believe because I grew up that way.")
But a "pure" agnostic neither believes nor doesn't believe in God. Rather, they think that God is outside the realm of human knowledge (or even human knowledgeability), and thus it doesn't matter what we believe about Him. They'd think that investing emotion or any sort of energy into God's (non-)existance is a waste of said energy, though they might find the resulting debate intellectually stimulating.
In practice, from a Christian point of view, atheists and agnostics may look quite the same, especially if they hold an if-you're-not-with-us-you're-against-us position. The problem is that the response to the Christian "missionary impulse" is quite the same. I keep joking that I'm an agnostic six days a week but turn into an atheist on Sundays.
Neither atheists nor (atheist or pure) agnostics allow God's word any authority, for example. Neither pray - although they might go through the motions.
Finally, while a theist's morals and ethics are perhaps bound up with the existance of God, an atheists or agnostics morals are not bound up with their respective believes. They will have independent ethical theories to guide them: compassion, rational choice, utilitarianism, whatever strikes their fancy. This means that, while God's existance is very relevant to the Christian believer on a daily-life basis, God's non-existance is rarely an issue for atheists and (especially) agnostics. It usually only comes up when they clash with "missionaries".
The more "missionaries" you meet, the more likely you'll turn your atheism into a active shield against these attempts. Agnostics may not really hold to atheism, but they could still use atheist discourse to counter the missionary impulse, simply because they are annoyed with the conversion attempts. So, basically, you might the same kind of response to your quesitons from both atheists and agnostics, with the difference that atheists mean them, while agnostics may be merely striking out in frustration.
The emotional baggage that comes with conversion attempts makes it hard for me to tell whether I'm an agnostic or an atheist. I honestly don't know. The thing is this: when I'm down, or drained of energy I really don't want to hear about God. Thinking about what God might mean takes energy I don't have to spare. Unfortunately, this is also when people are most likely to suggest "God" as a solution.
So here's the thing: if you want to convert me, bring up the topic when I have the strength to consider it, say, after we've watched a movie with relevant themes. Don't wait until I'm drained. I will have little patience for what you have to offer. No doubt I won't come across as open minded in my response. But, see, it's just bad timing.
I suspect that I'm, perhaps, an atheist agnostic. I maybe do believe that God doesn't exist, but I don't think that believe of mine matters much. It feels more like a question of taste: I don't eat bananas, I don't wear orange, I don't believe in God. But then again I haven't yet figured out what it is that I supposedly believe doesn't exist.