What is the difference between a plunger pump and a piston pump ?

Plunger Pumps and Piston pumps work on the same principle.  They both draw a fixed volume of liquid through the inlet and pressurize the liquid and then release the liquid.

The difference lies in the construction.  In Piston pumps, the seal which prevents leakage moves with the piston.  In plunger, the seal is stationary and the plunger alone moves.

This enables the generation of very high pressure.  Plunger pumps can build pressures of the order of MegaPascals.  Plunger Pumps are used in high pressure applications.