The word agnost means `not knowing'. An agnost is someone saying that he does not know, and is usually used in the context of knowledge about God, or the nature of reality. Agnosticism is the believe that it is impossible to know certain things for sure. There are different reasons for which one does not know, and for this reason we make a differnence between weak and strong agnostism.

Weak agnosticism

A weak agnost simple claims that he does not know whether God exists. This means that he does not exclude the possibility of knowing whether God exists.

Strong agnosticism

Strong agnoticism claims that it is impossible to know whether God exists. Somehow this is very strong claim, as it requires knowledge about reality to know this for sure. For this reason, some people hold the position that strong agnosticism is self-defeating.

A quote from An introduction to Atheism:

About the existence of God