You can think of a pulpectomy as a "partial" root canal. It involves involves accessing the pulp chamber of a tooth and removing infected and/or inflamed pulp tissue. As previous posters have mentioned, this is frequently performed as a palliative measure to relieve pain caused by the afflicted tooth until the root canal can be completed at a later appointment.
Root canal therapy involves the pulpectomy procedure, followed by cleaning and shaping the root canals, chemically disinfecting the inside of the tooth to kill infectious bacteria, and finally filling the canals with a sealing material to prevent them from becoming reinfected later.
Pulpectomy may be performed as a standalone procedure to relieve pain, but the root canal should be completed as soon as possible thereafter to prevent the recurrence of symptoms.