For simple systems with a relatively low level of pressure (about 140 to 180 bar or 14 to 18 MPa) the gearmotor is the most used type of hydraulic motors. The gearmotor is a very simple, reliable, relatively cheap and less dirt sensitive hydraulic motor. In the animation you can see that the direction of rotation is determined by the direction of the oilflow.
The pressure at the pressure side
is determined by the load (torque) on the shaft of the hydraulic motor.