Also jumper courses have big colorful jumps, with sharp turns, broken lines and roll backs. While hunter fences are more natural.. brown, green and white colors with bushes, flowers and shrubs. Hunter courses flow together more, with big wide turns and smooth lines. ALso jumper fences go higher than hunter fences...jumper fences can go up to 5'6" sometimes 7 feet, while hunter fences only go up to 4'6". Also in equitation classes the fences only go up to 3'6" in the highest levels.
I don't think the High Performance division is all that new, but I don't think it's all that common. Most of the shows I go to the typical working hunters (4") are the highest you see.
To the OP, on top of what everyone else has said... hunter courses can be fairly simple, esp when compared to a jumper course. You would not believe how many shows I've been to where the course is (regardless of height of division or prestige of the AA show): single on the quarter line, outside, diagonal, outside diagonal. OR... single on the quarter line, diagonal, outside, diagonal, outside.
Jumper courses will NOT have that kind of predictability! I can memorize 3 hunter courses in a second and jump one after the other without a blink of an eye. Jumper courses? Take some more time! The older I get the more important a course walk is to remember the course, not just figuring out how to ride it! Yikes!
(FYI Maura... I remember when it was AHSA as well ;) )