Hydraulic Basics
Hydraulic motors use a fluid--usually oil--to create motion. Hydraulic fluid is first pumped from a reservoir into pressurized tubes by a hydraulic pump, which is driven by an internal combustion engine. Those tubes take the fluid to the hydraulic motor or motors. The pressure turns the motors by flowing through them, then flows back into the reservoir where the whole process begins again.

Axial Plunger Motors
Axial plunger motors use a hydraulic piston to spin a rotor. Hydraulic fluid is pumped into a hydraulic cylinder, where it pushes a plunger out. That plunger turns the rotor. Once the plunger reaches the end of its motion, the fluid flows back out of the cylinder and the plunger retracts as the rotor turns the rest of the way around.