First thing I would do is get your hands on the organizing associatioins rule book. http://www.mainehorseassoc.com/12%20%20Rule%20Book.pdf
Some general guidelines -
There are three types of classes, hunters, where the *horse* is judged on style, manners, jumping form and way of going, equitation, where the *rider* is judged on position, form, effectiveness and control of the horse, and jumpers, where the judging is objective and strictly on time and jumping faults.
Short stirrup is usually an equitation class for very young riders, and there is usually an upper age limit of 10. Some organizations have adult beginner equitation called "long stirrup" to make the matter more confusing.
"Easy Does It" is hunter class at lower than recognized heights, in this case 18". Other names you'll see at other shows for classes of this type are Pre-Green, Baby Green, Schooling, Special, etc. If it has Hunter in the title, and not Equitation, it means the horse is being judged by hunter conventions.
Hunter Hack is a hunter class where the horses may be asked to hand gallop and jump 1 or 2 individual fences. Perfect for a horse not jumping full courses or with a questionable lead change.
A "Stake" class is one with a monetray prize, usually a set percentage of the entry fees returned to the winners. In some show organziations, Stake classes are more formal - you may see stock ties or even shadbellies.
HTH, and good luck!