Yeah, here are some wiki definitions as I am naff at explaining things :)
15 hand is just the height of the animal for the class - exceeding 143cm but noe exceeding 153cm
Workers (Working Hunter) - Working hunters can be of any of the weight classes, but are expected to jump a course of natural-looking fences in addition to performing the usual short show on the flat.
Should be shown with manes plaited (braided) and ears, faces and legs trimmed. Between nine and 13 plaits is traditional, although the number may vary depending on the horse's conformation. Tails may be pulled or plaited, although most professionals prefer to pull. Tack should be workmanlike, with bridles having a flat noseband and plain browband. Handlers should wear tweed jackets, shirt and tie, buff or canary breeches and black leather boots with garter straps.
Hunters should have a straight, ground-covering movement with little knee action. They should be able to show a good gallop, and come back to canter when asked. The horse should have straight, "daisy cutter" or "pointed toe" movement
that's probly more detail than needed but that explains a worker quite well, a horse for hunting :)