If you don't shout "I BELIEVE IN GOD", then you're an atheist.
Agnostic is the reason WHY you're an atheist - it just means that you believe the information to be lacking to really make a decision. (A Gnostic = No Knowledge/Information/Data)
So you could actually technically be an agnostic theist. You believe the information doesn't exist to logically decide, but you have enough faith to power through and believe anyway for illogical reasons.
So, agnostics ARE atheists. Up to and until the point when you say you firmly believe in god, you're an atheist. If you are an atheist BECAUSE there's no data, you're also an agnostic.
Most people have no idea about these definitions, though, so don't feel ignorant or belittled. There's just no vast atheist machine advertising this stuff.