I know you said it was an Open show, but the AQHA handbook will give you a good overview of what each class is generally
looking for. Your governing association may have slightly different ideas. AQHA Handbook
In a nutshell: Western Pleasure horses are bred & trained for a specific long, low & slow movement which will be shown as a rail class. There will likely be extensions of the gaits, but the focus is on presenting a horse who is cadenced, balanced, responsive & consistent. These are show horses and there will likely be some glitz & glamour, as presentation is part of the game.
Ranch Riding is for ranch bred type horses who may have a more forward gait, and they will do some sort of pattern with gait changes, poles to cross, spins/turns, and lead changes. The idea seems to be to showcase a working horses' ability to respond to typical maneuvers asked while working the ranch land. Tack and attire is more simple and work-oriented then WP classes, but still clean and put together. It is still a show class and your horse should be presented in as good a light as possible.
Have fun and good luck!