Piston pumps are reciprocating pumps consisting of a power end and a liquid end. The power end of the pump takes the driver power and converts the rotary motion to reciprocating motion. The driver can be a motor, an engine, or a turbine, whose output must then be converted by means of crankshafts, connecting rods, and crossheads, to reciprocating motion. An alternative, older design called direct-acting has a steam cylinder for the power end, with inlet and outlet valves to allow steam to directly drive the liquid end piston.

The liquid end of a reciprocating piston pump consists of a chamber having liquid inlet and outlet ports, with most designs having check valves in both the inlet and outlet ports. When the reciprocating piston strokes in one direction, the inlet check valve opens (while the outlet check valve remains closed), directing liquid into the liquid end of the pump. When the piston strokes in the opposite direction, the inlet check valve closes and the outlet check valve opens, allowing liquid to move into the discharge port and out into the system.

Piston pumps have the pistons operating inside cylinders, with the sealing taking place by means of elastomeric packing or O-rings on the outside of the piston. Alternatively, the piston itself may be made of an elastomer with an interference fit. Piston pumps can be single-acting or double-acting, although most are double-acting. With a double-acting piston pump, liquid is discharged during both the forward and return motions of the piston (requiring four check valves for each piston). A single-acting piston pump has liquid discharging only during the forward stroke of the piston. Double acting designs are generally associated with slower speeds and medium pressures, while single-acting designs are more generally associated with higher speeds and higher pressures.

Note that high inlet pressures are a very important consideration in the selection of a piston pump. Double-acting piston pumps are quite a bit more efficient than single-acting pumps when the inlet pressures are quite high. Double-acting piston pumps directly utilize the inlet pressure to reduce the power end load. Also, high inlet pressures can affect the selection of a prime mover. Double-acting piston pumps can use the pressure differential in the calculation of the horsepower requirement, whereas single-acting pumps can only use a fraction of the inlet pressure to offset discharge pressure.