According to the Oxford English Dictionary (OED) atheism is the disbelief in, or denial of, the existence of a god, while agnosticism is the view that the existence of anything beyond and behind the natural and material phenomena is unknown and (so far as can be judged) unknowable, and especially that a First Cause and an unseen world are subjects of which we know nothing.
People who don't take enough variables into account, often find themselves confusing the two. Occasionally, some "gnostic atheists" (those who believe "god does NOT exist") seek to include empirical agnostics into their midst by erroneously claiming "Agnosticism is a form of atheism and vice versa." THAT is patently UNTRUE.
Agnostics are empiricists who merely acknowledge that there are some things that appear humanly unknowable. They tend to hold this position because reason compels them to doubt the existence of any specific god/gods, yet they resist calling themselves atheists because they hold to that view tentatively.
Agnosticism is the position counter to Gnosticism - the belief that a human being can possess knowledge about a god. It's an epistemological term - about the possibility of knowledge in regard to 'god' claims - and not a statement about matters of belief. 'Agnostics' hold to the epistemological position that human beings can't actually know anything about something beyond nature, something theists call 'supernatural'. So they believe that there's no way for a human to know anything about a 'god' and there are many theists who agree with agnostics on that.
Theists can be agnostics, in fact, many theists say that they hold to their god belief on faith because they agree that we humans can't know things about the supernatural, or 'god'. Some very famous theologians have agreed that man is limited and that this means that man cannot have 'god knowledge". The list of theologians would include people like Martin Luther or Soren Kierkegaard.
It is this empirical doubt that separates agnostics from both Gnostics AND atheists, who, all too often imply that there is some (“scientific”) reason or some evidence to support the view “God does not exist,” whereas ALL empirical thought leads ONLY to the conclusion that “we DO NOT know.”