Answer: a hydraulic machine has water or air controlled by pistons that are airtight, water does great forces and air when compressed or squeezed to make things move, take a CAT machine for example (if you live in america, its those big yellow trebuchets or cranes) they use a piston for the arm and in front of those pistons you have mainly water (if your using a CAT machine) and those pistons push the water to and from your machines arm to create movement.
as you can see, in this picture, the CAT has hydraulics running against the arms of the machine