The word "hydraulics" comes from the Greek word ὑδϱαυλικός (hydraulikos) which in turn comes from ὕδϱαυλος meaning water organ which in turn comes from ὕδωϱ (water) and αὐλός (pipe).
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.