If you don't want to make a fool of yourself if you ever get involved in a conversation about beliefs or epistemology, it's important to understand this difference.
Agnostic - there are two ways to view this one. 1) You believe there are things in the world that are unknown, or 2) You believe there are things in this world that can not be known.
Atheism, broken down into "A - theist", in its essence refers to, the negative (the "a" bit, similar to "anti" or "un") belief in at least one deity (the "theist" bit).
Agnosticism is related to epistemological (about the nature of knowledge and understanding) ideas. For example, "are there things which can never be known".
So although these words are often used in the same context, they are on different levels. They come from to different topics and relate to different concepts within their own topics.
There are some people who believe that "people with no active belief in god" are agnostic, and not atheist. This is wrong. Only a person who has never heard of god, or the concept of a god truly has "no beliefs about god". Because once you ask someone the question "do you believe in god", the answer is either yes or no, which divides people into atheist or theist. This is based on the premise that a negative belief is still a belief. Only lack of knowledge about a topic can truly exempt people from a belief about it.
Also, someone can be "Agnostic", and believe that there are things in the world that can never be known, BUT believe that knowledge about a god CAN be known, and thus an atheist.
Most of the time, when people talk about agnosticism in the context of religion, they specifically mean "I don't know if god exists" or "I believe we can never know if a god exists", but this doesn't exempt them from a belief either way (theist or atheist). That's why it's called belief, because you don't have to be sure.
If you have no active belief about god, you're an Atheist. And you don't have to be scared.
The word "Atheist" isn't a scary label, it simply means what was described earlier. So don't be cautious about using this word. If it ever comes up, explain your position intelligently, you'll sound much smarter for it.