I have no experience with horses other than I think they are beautiful. I might be working on a horse farm this summer for my externship for school. I have a real dumb question.

What exactly is a thoroughbred horse? Is that the breed of horse itself, or is it like saying a thoroughbred dog can be a pug, rottweiler. Doe sthat make sense? lol. Like is a quarter horse or a clydesdale a thoroughbred because its pure blood? Or is it its own breed of horse.