curlysporthorse.com has an article about it and they put it in a very good clear way. i'll paste the article so you don't have to go looking for it! :-)
Hunter divisions at shows offer a variety of classes, but the gist of the competition is based on the form of the horse and/or rider. There are a variety of jumps set up in an arena and the rider is tasked with negotiating his/her horse over the course without refusals or knocking down a jump. The most fluid, flawless round is the winner. Hunter shows offer classes for horses and ponies, so unlike Dressage and Combined Training, ponies and horses don't compete against each other.
Jumper divisions may look similar to Hunter divisions to the average spectator, but there is a big difference. Jumper courses tend to be trappier, with higher fences, tighter turns, and they are often timed. A Jumper's form (or rider's form, for that matter) are inconsequential. What matters is that the horse and rider navigate the course with as few 'pulled rails' as possible. In Jumpers, the jump polls often sit in shallow "Jumper Cups" so that a slight rub will cause the pole to tumble. In some classes, if there is a tie after the first round, there may be a timed jump off, which is a shortened course, and the winner is determined by speed and accuracy. The horse with the fewest faults accrued at the end of the course is the winner.