Actuator is a arm, coil, or something like this which converts one energy to mechanical energy for releasing or closing gas or liquid. in other words we can say it is a type of special motor. The actuator can be electrical based, mechanical based, thermal based, hydraulic based or pneumatic based.
Electrical Based Actuator: electrical based actuators are solenoids which are used to control flow, it can be fluid or pneumatic flow. solenoid also control electrical energy.
Mechanical Based Actuator: Mechanical based actuators are hydraulic or pneumatic cylinders which control the flow of fluid or compressed air.
Thermal Based Actuator: Actuators which are actuated by thermally or magnetically are called thermal based actuators.