To clarify the definitions of atheist and agnostic, note that they both have the "a-" prefix. This "a-" prefix means "not" or "without," like in the words atypical, amoral, and asymptomatic. So the words atheist and agnostic stem from the words theist and gnostic.
What is the difference between an atheist and an agnostic?
Theist: From the Greek theos, meaning "god," theism at its most basic is the belief in the existence of a god. These days, theism is most commonly associated with monotheism, the belief in a single, solitary God.
Atheists, therefore, simply do not believe in the existence of God.
Gnostic: From the Greek gnosis, meaning "knowledge," gnostics believe in the redemptive power of knowledge. Gnosticism can be found in many corners of the religious world and is older than Christianity, but what all gnostics have in common is the belief that the only way to attain salvation is to discover the esoteric secrets of the universe through study and through various religious rites.
Agnostics believe that such ultimate knowledge can never be known, or at least not known with certainty. Generally, an agnostic is one who believes that we can never truly know whether or not God exists. Agnostics can be either theistic or atheistic, depending on what they believe (versus what they know).
One related philosophy is deism. Derived from the Latin word for "god," deus, deism is the belief in the existence of God on purely rational grounds, unlike theism, in which God is known through revelation. Especially during the 17th and 18th centuries, deists believed that God created the universe, set up its natural laws, and set it into motion but then took no further part in its function. For that reason, deism has been likened to a sort of atheism by some.