Cushing's is a metabolic disease, and I don't fully understand it myself, but it's caused by a tumor on the pituitary, and since the pituitary secretes hormones that regulate many body functions, when it's not working properly many things don't work properly. An extra long winter coat is one of the "side effects" of Cushing's, or if you rather one of the noticeable signs of Cushing's. It can also cause laminitis, wreck havoc with their immune system, cause them to be fat or extremely skinny (Jasper can go through both sides of that if his meds aren't right) and many other things.