Some people say that you can be both, some people say not. Agnosticism is usually defined as lack of knowledge, especially about god. But that comes in two forms, soft and hard, soft being, I do not know if god exists or not, and hard being, I am sure that nobody knows if there is a god or not.

Atheism however is usually, and you will find disputes about this, defined as lack of belief, especially in god. And it too is said to come in hard and soft forms, soft being, I do not believe in god, and hard being I believe there is certainly no god. ( Very like anti-theism another term sometimes used. )And that is why some people say that you can be both, because one is about knowledge, or at least belief about knowledge, and the other is about belief directly. Certainly soft atheism and hard agnosticism are compatible, but the other two maybe not.

It is complex, like many things in human culture, because you are trying to express truths about the nature of things, using language as a tool. And language is an undesigned tool, being little more than a collection of historical accidents, which means it is not always able to express things as we would like, even if we use it very well.

An agnostic believes that there might possibly be a god, however quite unlikely while an atheist does not believe there is any possibility of a god.