all pumps fall into two major groups: rotodynamic pumps and positive displacement pumps. Their names describe the method for moving a fluid. Rotodynamic pumps are based on bladed impellers which rotate within the fluid to impart a tangential acceleration to the fluid and a consequent increase in the energy of the fluid.  Positive displacement pumps force liquid to flow in volumetric proportion to decreasing pump volume. Hydraulic systems are a primary application, wherein the hydraulic fluid functions as the lubricant. Positive displacement pumps can be divided into reciprocating and rotary. Reciprocating pumps use pistons, plungers, or diaphragms to increase and decrease volume. Rotary pumps use a rotating device (gear, screw, or vane) to force liquid from the pump.

Also see compressor, pump.