There are three layers to a tooth:
The enamel is the hard part on the outside of the tooth and is actually the hardest part of the human body. The dentin is another hard layer inside the tooth. The pulp is actually the inner empty space where the nerve and tissue connects to the tooth.
The tissue of the tooth will become infected if that inner pulp is exposed. Some things that might cause the pulp to be exposed include a very large cavity or a fractured tooth. Once this occurs, the tissue is contaminated and gets infected. The treatment is typically a root canal.
The process of a root canal requires a dentist to clear out the dead or infected tissue so the area can be disinfected and sealed. This prevents more tissue from becoming contaminated and infected. A restorative dentist will then place a crown over the tooth to prevent future fracture.
This procedure is faster and less painful today than it was in the past. Advanced process, tools and materials have made it a lot more comfortable for patients.