Western Pleasure is an on the rail, walk, jog, lope. The judge is suppost to be judging how pleasurable a ride your horse gives you. They want to see your horse go around at the appropriate gaits with little to no visible signs of direction. They want them easily guided.
Horsemanship is a pattern and is different every time you do the class. They are judging you on how you ride your horse through the pattern and how good you are as a "horseman". If something doesn't work they want to see you fix it.
So in WP they are judging the horse, in Horsemanship they are judging you.
Western Pleasure is a much different class than Western Riding. Horsemanship is another class in itself.
You can read the rulebook of your particular breed's organization to get some familiarity of how each class operates.
Western Pleasure - Judged on the horse's way of going Western Riding - Judged on the horse's ability to perform a difficult pattern. All patterns are printed in the rulebook. These patterns contain a significant amount of lead changes, manuveuring between cones and over ground poles. Horsemanship - Judged on the rider's ability to get their horse to perform a pattern. The pattern is decided on by the judge & posted at the show at least one hour before the class competes. These patterns are usually made up of a simple series of gait transitions, stopping, backing, pivots, etc.