What is agnosticism? (A brief description)

a. One who believes that it is impossible to know whether there is a God.

b. One who is skeptical about the existence of God but does not profess true atheism.

• Agnosticism is the view that the truth values of certain claims – especially metaphysical and religious claims such as whether or not God, the divine or the supernatural exist – are unknown and perhaps unknowable. According to the philosopher William L. Rowe: "In the popular sense of the term, an agnostic is someone who neither believes nor disbelieves in the existence of God, while a theist believes that God exists, an atheist disbelieves in God"

See also: 

http://atheism.about.com/od/aboutagnosticism/a/what.htm