Fluid power is the technology that deals with the generation, control, and transmission of power, using pressurized fluids.
For examples: fluid power steers and brakes automobiles, launches spacecraft, moves earth, harvests crops, mines coal, drives machine tools, controls airplanes, processes food, and even drills teeth.

Fluid power is called hydraulics when the fluid is a liquid and called pneumatic when the fluid is a gas. Thus fluid power is the  general term used for both hydraulics and pneumatic. The first hydraulic fluid to be used was water because it is readily available. However, water has many deficiencies in comparison to hydraulic oils. For example water freezes more readily, is not as good a lubricant, and tends to rust metal components. In spite of these deficiencies, there is a renewed effort underway to return to water in certain applications because of water’s abundance, non flammability, and environmental cleanliness. When water hydraulics is used, the water contains additives to improve lubricity and rust protection and prevent freezing where necessary. 

Hydraulic oils are currently much more widely used than water, but as environmental concerns continue to become more serious, water hydraulics is expected to become more prevalent.