Hydraulic external gear motors are used in simple rotating systems. Their benefits include low initial cost and high rpm.
A hydraulic gear motor consists of two gears; the driven gear (attached to the output shaft by way of a key etc.) and the idler gear. High pressure oil is ported into one side of the gears, where it then flows around the gear edges, between the gear tips and the wall housings in which it resides through to the outlet port. The gears then interlock, not allowing the oil from the outlet side to flow back to the inlet side.