Hey justagirl. They pretty well covered your first question. As for Navicular Disease you will see it most often in running or jumping horses. But is often caused long toes and short heels. What happens is the flexor tendor(which you can feel behind the fetlock joint) is over extended it puts too much pressureon the navicular bone. A horse with ND will usually stand with their foot pointing out in front of there body with the heel off the ground a little.
It can be a rough deal for the life of the horse. On the shoeing part stand the horse up as much as possible and use wedge pads and heeled shoes. That will relieve the foot. Some people recomend heels for life.
Do you have a Navi. horse? There heels will usually be contracted inward, sometimes nearly touching at the back.
As for ringbone I hardly ever see it. If its a low towards the hoof it usually wont cause longterm lameness. Now if its up high and affects the pastern joint you will have trouble ever getting the horse sound.
Hope this helps.