Agnosticism is a belief that knowledge of the existence or non-existence of God is impossible. It is often put forth as a middle ground between theism and atheism. Understood this way, agnosticism is skepticism regarding all things theological. The agnostic may generally hold that human knowledge is limited to the natural world -- that the mind is incapable of knowledge of the supernatural.
From a Christian perspective, some would see agnosticism as a denial, like atheism, since natural revelation reveals to all, Christian or not, that there is a God, and special revelation, i.e. God's Word in the Bible, reveals the particular Christian, Triune God.