Endodontic therapy, better know as a root canal, is necessary if a tooth has become constantly painful either due to decay, trauma, previous treatment or other circumstance. In this procedure the nerves is removed form the inside of the root of the tooth. This allows the rest of the tooth to remain intact and continue functioning. Once the nerve is removed from the tooth, pain will cease rather quickly.
Blood vessels are also removed from the tooth during a root canal. That vein the case, it can become dry and break. Therefore a crown is almost always recommended as well.