God is the uncaused first causer.
He/It is the unexplainable beginning to the cause and effect nature of the universe and also a very unsatisfying end to the question of who or what created everything. It's a philosophical cop-out.
Note: It has also be defined as the unmoved first mover, the unperceived first perceiver and so and and so forth. I cannot remember who originally came up with this term but many people have since adopted it.
Note: By 'God' I mean whatever god, gods, mystical forces, non-mystical forces, or whatever you subscribe to. Semantics, bah.