What are the difference between over-hand serves and underhand serves?

AN over hand serve is when the ball is usually thrown then hit while staying above the servers head, the hand is carried back and over the head to hit the ball. AN under hand serve is when a player holds the ball out, and hits the bottom of the ball with the other hand, this motion would look like and regular underhand toss. The overhand serve is used more at the start of the high school level all the way to the professional level. An underhand serve is very beginner.