A track day is different than autox - autox is around cones, on a "temporary" course.
A track day is at a race course. Here is a lap during a track day at Hallett:
http://www.youtube.com/watch?v=SOdtrhSyJlk There are probably designated "passing zones" on the straightaways, no "door to door" racing, and multiple cars on track at the same time.
Nearly all cars can sign up and drive. (Safety check, etc.)