A specialist in dentistry is a master of a specific field in dentistry. After receiving a dental degree, a general dentist can choose to specialize through additional schooling. Depending on the field, specializing may require two to six years beyond the four years required for a general dental license.
The fields in dentistry that offer specialization are:
A general dentist has received training and holds a license to perform all treatments listed above (except general anesthesia). However, all types of treatments vary in complexity. There is a "line," based on complexity of treatment, that general dentists draw based on their experiences and judgement. Any treatment with complexity that exceeds that line will be referred to a specialist if the general dentist believes the outcome of the treatment will be improved in the hands of a specialist.
For example, many general dentists perform root canal treatments. But there are some teeth which pose unusual difficulty (curved roots, calcified canals). A root canal specialist, an endodontist, has experience dealing with these unusual circumstances. The chances of success of the root canal treatment is much higher if performed by the endodontist. In this case, a general dentist will not perform the treatment. Instead, they will refer the patient to an endodontist.
Some procedures are best left to the specialists, and it takes a humble and perceptive general dentist to know where to draw the line.
Similar examples can be shown for all specialties.
And similar analogies can be made in medicine. For example, a heart surgeon generally will not perform brain surgery for the same reasons listed above, although both hold surgical licenses.