Endodontics is the branch of dentistry that deals with treating diseases or injuries of the dental pulp. Common causes of pulp inflammation or infection are cracked/chipped teeth, a deep large cavity/filling or other more serious injuries to the tooth.
Root canal therapy is the physical removal and chemical disinfection of the nerve area or the nerve tunnel thru the center of the root. Depending on the tooth, the tooth may have 1-4 or more nerves that need to be addressed. The empty space is then filled with a bio-static material to prevent bacteria from returning and causing a new infection. There are times that a re-infection does occur and requires additional retreatment (aka: redoing root canal therapy).