Dentistry is the branch of medicine that is involved in the study, diagnosis, prevention, and treatment of diseases, disorders, and conditions of the oral cavity, the maxillofacial area, and the adjacent and associated structures, and their impact on the human body. 

To the layman, dentistry tends to be perceived as being focused primarily on human teeth, though it is not limited strictly to this. Dentistry is widely considered necessary for complete overall health. Doctors who practice dentistry are known as dentists. The dentist’s supporting team which includes dental assistants, dental hygienists, dental technicians, and dental therapists aids in providing oral health services.

Prevention
Dentists also encourage the prevention of oral diseases through proper hygiene and regular, twice yearly, checkups for professional cleaning and evaluation. Conditions in the oral cavity may be indicative of systemic diseases such as osteoporosis, diabetes, or cancer. Many studies have also shown that gum disease is associated with an increased risk of diabetes, heart disease, and preterm birth.

Source: Wikipedia