This is a Linear Actuator, which creates motion in a straight line, in contrast to the circular motion of a conventional electric motor. Used in computers peripherals , disk drives and printers  wide variety of applications on Robotics projects. Used in valves and dampers. Could be coupled with outer gear system to vary the torque and speed. An electronic speed control could be added to vary the speed of linear movement. Available  in 4, 8, 12, 18, 24 volts. Please specify the shaft length, and torque requirement.