What sport is the movie "Days of Thunder" about?

Tom Cruise was riding high after "Top Gun" turned him into the king of need-for-speed movies, so naturally Hollywood wanted to stick him behind the wheel of something else fast and dangerous. "Days of Thunder" hit theaters in 1990 with Tony Scott directing again, Nicole Kidman as the love interest, and enough high-octane action to make your ears ring. But instead of fighter jets screaming across the sky, this time Cruise was tearing around oval tracks at breakneck speeds. What sport is "Days of Thunder" all about?