Well, jumpers is basically the discipline for horses and riders who may lack the movement and form for flat and hunter classes.
Umm... or it's for horses that jump well and are successful at navigating a more technical course... Yes, some jumpers are hunter rejects, but jumpers are a whole sport in and of itself. And unlike hunters, there is no height limit. It takes a special and very talented horse to be successful in it.
First and foremost, when it comes to rules you need to understand the "tables". (assuming you're showing at a USEF sanctioned show). Here is an earlier post where I described them... Jumper Tables?
Know what your tables are so you know how to ride the course.
Our local shows have hunter classes and then jumper classes, and often they're in the same ring and the course does not get reset. (At rated shows the jumpers get a whole ring unto them selves and the course gets reset about every other level). Since the jumps are set in the same places that the hunter courses are set, this means that the jumper courses are either 1) basically an eq course or 2) made up by someone who isn't a jumper and they just throw in some really weird turns. At rated shows thought they have course designers that are making courses specific to the discipline and i don't thin ktheyre going to be too different from eventing show jumping. Your horse must be brave (the jumps will be more brightly colored and often weird shapes, NOT something you'd see on a hunt course!), the strides aren't necessarily set on a 12 foot stride so they must be adjustable, and as you get higher up they will throw in some tricky elements like combinations (one stride, a triple combo, etc) and difficult turns.
My biggest pet peeve are people who just tear around the ring as fast as they can making everyone gasp. While they may be successful at the lower levels their lack of ability to ride a technical course will bite them in the butt as soon as they jumps go up. So even though Sandy doesn't like dressage, i'd definitely start working on her rideability. Get her back on her haunch so she can really turn well, get her moving laterally, extra responsive and able to lengthen and shorten her stride and pace, etc. I'm a hunter princess at heart and speed is not in my natural vocab but I've been showing a lot of jumpers lately and i do have to say it's been fun. :)