An actuator is a mechanical device for moving or controlling a mechanism or system. The actuator is usually a physical mechanism but also refers to an artificial agent intelligent agent.

Examples and applications
Examples include:

Mechanics - pneumatic actuators, motors, hydraulic cylinders, etc.
Human - Arms, hands, fingers, legs
Mail transfer agent - Update software
In engineering, actuators are frequently used as mechanisms to introduce motion, or to clamp an object so as to prevent motion. In electronic engineering, actuators ACTT, are a subdivision of transducers. They are devices which transform an input signal (mainly an electrical signal) into motion. Specific examples are Electrical motors, pneumatic actuators, hydraulic pistons, relays, comb drive, piezoelectric actuators, thermal bimorphs, Digital Micromirror Devices and electroactive polymers.
Motors are mostly used when circular motions are needed, but can also be used for linear applications by transforming circular to linear motion with a bolt and screw transducer. On the other hand, some actuators are intrinsically linear, such as piezoelectric actuators.
In virtual instrumentation actuators and sensors are the hardware complements of virtual instruments. Computer programs of virtual instruments use actuators to act upon real world objects.