Who are the judge's?
Usually, it's a combination of the organizers and a few respected game developers who come together. We usually don't disclose this for privacy reasons and because this can change.
As Colin says, judging is informal - we just shortlist down to a few frontrunners and pick based on how fun / original the game is, how well it's put together and how well it incorporates the theme. No points or silly things like that. It's relaxed but thoughtful.
As has been said, the point of the jam is to get the community together to make some cool games. Just have fun and find the fun, right?