Agnosticism is the view that the true value of certain claims – especially metaphysical and religious claims such as whether God, the divine or the supernatural exist – are unknown and perhaps unknowable.

Atheism is, in a broad sense, the rejection of belief in the existence of deities. In a narrower sense, atheism is specifically the position that there are no deities. Most inclusively, atheism is the absence of belief that any deities exist. Atheism is contrasted with theism, which, in its most general form, is the belief that at least one deity exists

So Atheism means you believe that a god does not exist, and Agnosticism means you don't believe in a god.