Agnostics claim either that it is not possible to have absolute or certain knowledge of God or gods; or, alternatively, that while individual certainty may be possible, they personally have no knowledge.
Atheists have a position that either affirms the nonexistence of gods or rejects theism. When defined more broadly, atheism is the absence of belief in deities, alternatively called nontheism. Although atheists are commonly assumed to be irreligious, some religions, such as Buddhism, have been characterized as atheistic because of their lack of belief in a personal god.
Agnostic means you are uncertain of your beliefs. It tends to imply you believe there is something, but you don't know what. Atheists operate under the assumption or belief that there is no god. It tends to imply that the universe is entirely natural and that humanity is the most advanced natural product that has been on Earth.
Short Answer: a means no + ancient Greek, theos, which means god = atheist, atheism, etc. = no gods or no God or both. Again, a, no + ancient Greek, gnosis, which means knowledge = agnostic, agnosticism, etc. = no knowledge about gods or about God or both. Similar to diagnosis or through knowledge; and prognosis or future knowledge, in medicine. However, each atheist, and each agnostic, is unique (as we all are) and each doesn't believe, or believes, at least, a little bit differently, from every other one, so you can't give one definition for one or the other, that will apply to every individual atheist or agnostic. I'm a practicing Roman Catholic Christian, and definitely, a believer, in case you want to know. Hope this helps; and doesn't just muddy the waters for you. Glad you asked this question for yourself and everyone else who wondered about the same thing! Frank