Why was/is JFK sometimes referred to as Jack as well as John?

I've seen doentaries about JFK where in one instance he's referred to as John then later as Jack. This has happened in more than one doentary regarding JFK so it has left me somewhat confused. Even the mighty Wikipedia lists him as John Fitzgerald "Jack" Kennedy but does not mention why. So I turn to you, the collective, why is this, where does the Jack come from?