An orthodontist has 2-3 years of specialized education beyond dental school and is a specialist at straightening teeth and insuring proper form and function. This is a brief description of what the American Association of Orthodontists website reads. Let me put it into more practical terms. We relate dental structures to the jaws and then in turn relate the jaws to the cranio-facial structures of the patient. There are a number of ways to analyze these dimensions which make the specialty challenging and fun. In addition to analyzing them we as Orthodontist make the diagnosis and put in a plan to achieve and optimum aesthetics, function, and balance. This takes a years to master. An Orthodontist will only practice Orthodontics which is why many agree to see a specialist in Orthodontics when a patient merits treatment for “braces”.
To get back to the American Association of Orthodontists take on it:
Orthodontists receive more formal education than dentists to specialize in straightening teeth. Like dentists, orthodontists graduate from dental school. Then, to be an orthodontist, it takes an additional two to three academic years of education in an accredited orthodontic residency program. Orthodontists are dentistry’s specialists in straightening teeth and aligning jaws to create optimal function and form. Orthodontists only practice orthodontics. They treat hundreds of patients a year, drawing on tried-and-true and new orthodontic appliance technologies to get patients to the best results. Orthodontists have knowledge of the full range of orthodontic appliance “tools”—including braces, clear aligners and other orthodontic devices. They know what to use and when because they work with these tools every day. Orthodontists build on their knowledge of orthodontics through on-going continuing education in orthodontic technology and practice.
I hope this helps!