The basic idea is very simple: force that is applied at one position is transmitted to another position using an incompressible fluid, like oil, which cannot be compressed but rather is displaced when pressure is applied.

A reservoir, a hydraulic cylinder, and a pump make up the three main parts of a hydraulic pump. Pumping hydraulic fluid from the reservoir into the bottom cavity of the cylinder causes the piston rod to be pushed up, which pushes the fluid in the other cavity back into the reservoir. This procedure pressurizes the chamber and extends the piston to its full length. Pumping hydraulic fluid into the upper chamber of the cylinder pushes the piston rod down, pushing the oil in the other chamber back into the reservoir, completing the cycle.

Axial piston pumps - are positive displacement. There are several pistons in a circular arrangement in a cylinder block.

Radial piston pumps - a circular arrangement of hydraulic pistons, and valves which allow pressures greater than 650 Bar (~10,000 psi) to be built.

Screw pumps - positive displacement pumps. Have one or more screws that rotate to transfer high or low viscosity fluids along an axis.