A valve actuator is a mechanical device that uses a power source to operate a valve. This power source can be electric, pneumatic (compressed air), or hydraulic (the flow of oil). There are two main types of actuators, one for each of the two main types of valves that require them. They are rotary and linear.
A ball valve is a form of quarter-turn valve which uses a hollow, perforated and pivoting ball to control flow through it. It is open when the ball's hole is in line with the flow and closed when it is pivoted 90-degrees by the valve handle.
In a mechanical device, an actuator is a component that turns the control signal into movement. Examples of actuators include: Electric motors.