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 (by USEF and usually USHJA) 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, more prize money, and medal qualifying classes that the smaller shows won't so they usually have more (thus more difficult!) competition, fancier horses, etc. And of course more expensive! I think the B and C shows tend to vary from region to region! Our B shows are fairly similar to A shows, but maybe not quite as competitive. Schooling shows tend to be much more casual, but again depend on where you are! Ours also have more beginner level classes since that's the majority of the crowd.