Head – is the maximum height that the pump can achieve pumping against gravity. 
Rate of Flow – the total volume throughput per unit of time at suction conditions.  Capacity is another term.
Net Positive Suction Head (NPSH) – the minimum pressure required at the suction port of the pump to keep the pump from cavitating. 
Impeller – The bladed member of a rotating assembly of the pump which imparts the principal force to the liquid pumped.
Casing – the portion of the pump that includes the impeller chamber and volute diffuser.
Volute – is the casing that receives the fluid being pumped by the impeller, slowing down the fluid’s rate of flow.  A volute is a curved funnel that increases in area as it approaches the discharge port. 
Diffuser – A set of stationary vanes that surround the impeller.  The purpose of the diffuser is to increase the efficiency in the pump by allowing more gradual expansion and less turbulent area for the liquid to reduce in velocity.
Inducer – A single-stage axial flow helix installed in the suction eye of an impeller to lower the NPSH.
Centrifugal Pump – produce a head and a flow by increasing the velocity of the liquid through the pump with the help of the rotating vane impeller.  Centrifugal pumps include radial, axial and mixed flow units.  All centrifugal pumps include a shaft-driven impeller that rotates inside a casing.  Liquid flows into the suction inlet of the casing and is thrown to the outside of the casing, then exits the discharge port.  The velocity imparted to the liquid by the impeller is converted to pressure energy or “head”. 
Positive Displacement Pumps – draws fluid into a compartment at the inlet and moves it to an outlet for discharge, most typically using a rotary, reciprocating, or diaphragm method to move fluid.  PD pumps will move fluid at the same speed regardless of the pressure on the inlet end whereas centrifugal pumps will not.