Shuttle Hydraulic System

This drawing depicts the parts of an Orbiter which run on hydraulic systems.

According to HowStuffWorks:  "The basic idea behind any hydraulic system is very simple:  Force that is applied at one point is transmitted to another point using an incompressible fluid. The fluid is almost always an oil of some sort. The force is almost always multiplied in the process." 

See the graphics in this link to better understand how hydraulic systems work.   

Credits

Drawing, NASA.

Quoted passage on hydraulic systems, HowStuffWorks.com.