akinkysnorlax did a great job explaining what a Super User is.
I don't believe there is a definite answer on how to become one. It's some combination of forum activity(post count), helpfulness(thanked count) and visibility. As far as I know there isn't any sort of magic number on anything. You can have 2000 posts but not be a helpful and contributing member. Or you can be extremely helpful yet have a low thanks count.
I think ultimately it's a judgement call by Matt. He's active enough here that he can tell who the "Super Users" are. I also imagine that it's not something he constantly updates which is why you will see people labeled in batches.
I don't speak for everyone, but personally I was super excited when I went to a thread I posted in and happened to notice my name was orange out of the corner of my eye. I can tell you that my wife did NOT understand my excitement when I explained to her how I got labeled a Super User on the forum I frequent.