All purpose saddles have a deeper seat than most jumping saddles and a more forward flap than most dressage saddle. The idea is that they are versatile for the rider who wants more of a pleasure saddle.
Trail saddles are similar to all purpose with respect to a moderately deep seat and slightly forward knee roll to allow versatility of position. They have the added feature of a more padded seat for long distances and more hardware for attaching items necessary for long distance riding.