I don't know if it is different by state but here in NC there are A shows and B shows. I believe they are hunter based classes, maybe also jumping. Anyhow I would like to know the difference if anyone can tell me. Thanks.
The letters are for the ratings of USEF recognized shows. It goes from AA, A, B, and schooling shows which are C. The AA and A shows are nationally recognized very competitive shows, with people coming in from all over the region, and sometimes all over the country. They will offer higher levels of classes and medal qualifying classes that the smaller shows won't and usually have more (thus more difficult!) competition, fancier horses, etc. And of course more expensive! The classes are either hunter or jumper type classes divided by differing levels of divisions. Within the hunter divisions (hunter is based on the horse) you also will have Equitation classes (based on the rider). So at most shows for the hunter divisions you'll have a flat class based on the horse (undersaddle), a flat class based on the rider (eq), a couple jumping classes based on the horse, and a couple equitation over fences (based on the rider). I think that's pretty standard across the board. That might be more information then what you were looking for.... ;)
Upnover pretty much covered it. I'm pretty sure the USEF ratings are just determined by the amount of prize money being offered. As a result, the A shows are more expensive and draw a higher-quality crowd. Anyone can compete at any of these shows; you may have to be a USEF member. You do not have to get points or placings in the B and C shows to compete in an A show though.