It's my own voice in my head. Its difficult to determine on what level I control it. It doesn't "make" me do things, but it's not like I decide what it says before it speaks. I often wonder if it's the other half of my brain. I speak differently in person than I do inside my head, or type, for that matter.
Regardless whether it's a kind of supplemental personality inside my head, one thing I'm convinced of is that many people mistake their internal monologue for the Voice of God.