Dentist have a degree and then attend 4 years of dental school where as dental hygienist only need a two year associate degree in dental hygiene.
Dentists come in after to diagnose any problems, examine x-rays, repair teeth, prescribe medicine, perform procedures like root canals, fillings for cavities, dental crowns and much more. Dentists are able to provide more skilled treatment whereas dental hygienists provide more general teeth care.