Agnosticism can be either theistic or atheistic.
Straight from the man who coined the term (Thomas Huxley):
No better to explain this word then the man who coined the term, Thomas Huxley (1869). When Huxley was looking for a way to describe his thoughts, he selected the early religious sect known as "Gnostics" as a prime example of men who claim knowledge of the supernatural without justification and he distinguished himself as an "agnostic" by stipulating that the supernatural, even if it exists, lies beyond the scope of human knowledge. We cannot say if it does or does not exist, so we must suspend judgment.
Agnosticism signifies the impossibility of knowledge in a given area. An agnostic is a person who believes that something is inherently unknowable by the human mind.
The term "agnostic" does not, in itself, indicate whether or not one believes in a god. Agnosticism can be either theistic or atheistic.
The agnostic theist believes in the existence of god, BUT maintains that the nature of god is unkowable.
The agnostic atheist maintains that any supernatural realm is inherently unknowable by the human mind and the existence of any supernatural being is unknowable as well.