Actuator is a mechanism of turning the Energy into Motion

They can be categorized by the source require to generate the Motion.

(a) Pneumatic Actuators (Air).
(b) Hydraulic Actuators (Water and Air).
(c) Electric Actuators (Electrical Energy).
(d) Thermal Actuators (Heat).

Pneumatic Actuators : Mechanism that turns compressed air into motion. Example Tie rod cylinders, Grippers etc.

Hydraulic actuators: uses liquid energy for the motion. Examples Hydraulic Breaks etc

Electric Actuators: Turns Electric energy into motion. Example electrical motors, parallel plate, combo drives etc

Thermal Actuators: These are used to convert the thermal energy into motion. Examples VAV Diffuser.