Stolen from google,
Harness/bridle -- basically the same vegetable tanned cowhide, but the bridle may have a different stuffing (oils and fats) than harness depending on the tanner. And there's the rub, or at least part of it: some terms are somewhat flexible in their use and not strictly standardized, at least in part because the recipes and techniques are often trade secrets. One tanner's harness might be another's bridle, just (generally) thicker.