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.
Primary (Baby) teeth in children have relatively large pulp spaces and a cavity does not have to get very large before it reaches the pulp chamber.
When the soft tissue in the pulp chamber is infected (has bacteria in it) or affected (is inflamed), it can be removed by a dentist under local anaesthetic. If the soft tissue in the canals is still healthy enough, a special medicated filling can be put into the chamber in an attempt to keep the remaining pulp (in the canals) alive. The process of removing the pulp from the chamber is the actual "pulpotomy", though the word is often used for the entire process including placement of the medication.
Afterwards the tooth is restored with a regular filling or a stainless steel crown.
A pulpotomy can be done to both permanent and primary teeth.