Hydraulics is a branch of science and engineering concerned with the use of liquids to perform mechanical tasks.
Typically, the fluid used in a hydraulic system is an incompressible liquid such as water or oil. Pressure is applied by a piston to fluid in a rigid vessel, causing the fluid to press on another piston that delivers energy to a load. If the areas of the two pistons are different, then the force applied to the first piston will be different from the force exerted by the second piston. This creates a mechanical advantage.