A linear actuator is used to generate controlled physical linear displacement. Mechanical actuators typically convert rotary motion of a control knob or handle into linear displacement via screws and/or gears to which the knob or handle is attached. A jackscrew or car jack is a familiar mechanical actuator. Rotation of the jack handle is converted mechanically into the linear motion of the jack head.

Advantages

Electrically operated designs can be self-contained, requiring only electrical power and a control interface (which can be as simple as a button or toggle switch).
They can be operated in parallel or series, using multiple actuators to perform a single task.
They can be very precise, some designs use servo motors and/or specialized sensors to accurately measure displacement.
They can be made very fast compared with alternative technologies, (with direct drive type typically 10 times faster than ball screw drives).
They are relatively cheap and easy to work with compared to other methods of providing linear motion.