One of advantages of using machines to control processes for you is that you can harness tremendous amounts of power and energy which humans do not possess.
By a flick of a switch, huge sliding flood gates can move to allow large quantities of water to flow out of a dam. With just a slight twist on a handle, supertankers weighing hundreds of thousands of tons are able to turn because of corresponding rotation of huge rudders.
Similarly, processes operating valves can be remotely operated to control flow, temperatures, pressures and so on.
All these final control elements - flood gates, rudder, valves - must be operated by actuators. The function of actuator is to move to control elements in processes.
In order to move actuator, there must be a medium for transmitting required power or action. All actuators are mechanical in nature. The common means of transmitting power are: pneumatic (air), hydraulic (fluid), and electrical (motors).
The actuators must be designed to work for that medium. There are several advantages and disadvantages for choosing a particular medium. For example, pneumatic actuators are particularly useful when atmosphere contains flammable gases. By using pneumatic actuators, there is very little chance that a spark can be generated to cause a fire. However using air for actuating may be more expensive to install and to maintain.