Agnostic and atheist are two terms that refer to a person's belief or
disbelief in God. Atheist exclusively refers to someone who does not
believe in the existence of God. Some prominent atheists completely
reject the mentioning of God on money and refuse to have their children
state the pledge of allegiance to the American flag because of the
phrase "one nation, under god."
By contrast,
agnosticism does not necessarily make any claim about whether or not a
person believes in God. Instead, an agnostic believes that it is not
possible to prove or disprove the existence of god. There are agnostics
who are theists, that is, people who believe in God; there are also
agnostic atheists, who do not believe in the existence of God.