The mandible or the lower jaw bone behaves as a ring. As much as you try, it is almost impossible to break a glass bangle (a ring-like object) in a single place - it always breaks at the diametrically opposite segment. Similarly, the mandible tends to break in two places when injured. When it does not break in two places, injury similar to a fracture has been absorbed by the temporomandibular joint, which compromises the joint and its function significantly.
The weak points on the mandible are where the nerves enter and leave its substance: at the entrance of the inferior alveolar nerve, and at the exit of the mental nerve. When the fracure passes through a tooth root, it is likely to be "compound" in nature, meaning that it communicates with the mouth. Here, the risk of infection is very high, and untreated fractures not only malunite, but may never unite, and lead to life threatening neck space infections.
Mandibular fractures require individualised treatment, dependant on the age of the patient, the dentition, the oral hygiene, and the timing of surgery (how distant to the injury).
Most fractures can be reduced and fixed in place (titanium plates and screws are employed, so there is no risk of "rejection", and little risk of infection) from the inside of the mouth, avoiding any scars. However, those fractures that happen just below the head of the mandible(the subcondylar fractures) will require external approaches, as would certain other types of fractures.
Most patients will then be permitted soft, mushy food for 4 weeks, after which a fairly normal lifestyle may be resumed.
The defining factor of how well the procedure has been will be very obvious from the "occlusion" of the patient, which is the resting position of the jaws in relation to each other: the patient will be able to sens the return to normalcy soon after surgery.