A pulpotomy is the removal of a portion of the pulp, including the diseased aspect, with the intent of maintaining the vitality of the remaining pulpal tissue by means of a therapeutic dressing.
A healthy tooth has a space inside it called the "pulp space" which is filled with soft tissues - nerves, blood vessels and pink connective tissue. If a tooth gets a large cavity, the bacteria in the decay can damage the pulp, which is often what causes toothache.
Damage to the pulp of permanent teeth usually requires a Root Canal Treatment or Endodontic Therapy. The pulp of primary or deciduous teeth, which only have to survive until an adult teeth come in, and because they have a better blood supply, can sometimes be saved.