An MD is a medical degree. It's possible to have an MD but not practice medicine professionally in the same way someone can earn a law degree (a JD) but choose

An MD is a medical degree. It's possible to have an MD but not practice medicine professionally in the same way someone can earn a law degree (a JD) but choose to not be a practicing lawyer. "Professor" is a academic title for people at a university or advanced teaching institution. Most professors do have terminal degrees in their respective fields like PhDs, MDs, or JDs, so you could also call a Professor with a PhD "Dr. X" instead of "Prof. X". But, "Professor" is generally considered a more prestigious title since its much rarer and harder to get a professorship than get an advanced degree.