What is the difference between hydraulically powered and pneumatically power robots?

Hydraulics is the use of fluids, usually oil or other hydraulic fluids, that are used under pressure to move a piston from the inside of a cylinder to the outside of the cylinder. If you attach this to a lever or tool, it can be used to cut, fold, crimp, move, punch an item or many other tasks. By reducing the pressure in the cylinder, the piston will retract back inside of the cylinder, thus retraction the tool or device it is attached to.

Preumatics does the same action or function by using air under pressure instead of using a fluid.