A registered orthodontists is a fully qualified dentist who has studied for an additional three years of a full time Masters Degree in orthodontics. This qualifies them to diagnose and treat disorders concerning straightening of the teeth and the alignment of the jaws.
A dentist is concerned with treating disorders and diseases of the teeth and mouth. They can sometimes undergo some further study in the area of orthodontics but do not have the depth or breadth of knowledge that an orthodontist does. An orthodontist is the best person to treat disorders concerned with the alignment of the teeth and jaws.