This description never was clear to me either. The Pump Handbook, 3rd edition by Karassik has the following explanation:
"Vertical turbine pumps were originally developed for pumping water from wells and have been called deep-well pumps,turbine well pumps, and borehole pumps. As their application to other fields has increased, the name vertical turbine pumps has been generally adopted by manufacturers. This is not too specific a designation because the term turbine pump has been applied in the past to any pump employing a diffuser. There is now a tendency to designate pumps using diffusion vanes as diffuser pumps to distinguish them from volute pumps. As that designation becomes more universal, applying the term vertical turbine pumps to the construction formerly called turbine well pumps will become more specific."
