In the popular sense of the term, an "agnostic", according to the philosopher William L. Rowe, is someone who neither believes nor disbelieves in the existence of God, while a theist believes that God does exist and an atheist does not believe that God exists.

Ignosticism or igtheism is the idea that every theological position assumes too much about the concept of God and other theological concepts; including (but not limited to) concepts of faith, spirituality, heaven, hell, afterlife, damnation, salvation, sin and the soul.

Ignosticism is the view that any religious term or theological concept presented must be accompanied by a coherent definition. Without a clear definition such terms cannot be meaningfully discussed. Such terms or concepts must also be falsifiable. Lacking this, an ignostic takes the theological noncognitivist position that the existence or nature of the terms presented (and all matters of debate) is meaningless. For example, if the term "God" does not refer to anything reasonably defined then there is no conceivable method to test against the existence of god. Therefore, the term "God" has no literal significance and need not be debated or discussed.
Source: WIKIPEDIA